On the Structure of the Excretory Organs of Amphioxus. Part 2.—The ...

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190 EDWIN S. GOODRICH. focussed to the lower surface; the nuclei of the wall are again visible. There is no opening. Innumerable figures could be given of series of sectious all telling the same story. But the critic will say : if the diverticula are really closed, sections taken at right angles through their tip should show the tubes cut across embedded in the thickness of the wall. Such sections are not difficult to find, and I figure several on Plates 12 and 13. Figs. 10 and 12 represent two consecutive sections across the tip of a branch. In the fh'sb are seen the tubes entering the wall, while the next (fig. 12) strikes the lumen. A small part of this figure is shown slightly diagrammatised (fig. 11) on a larger scale. Again three consecutive sections are drawn in figs. 15, 16, and 17. Here two sections cut through the solid wall before the lumen is reached. Lastly, fig. 18 represents a section through two adjacent processes, one of which has been cut so as to expose the lumen, while the other shows very clearly the soleuocyte tubes piercing the wall aud embedded in its cytoplasm. The evidence of all these sections is quite unequivocal; it would serve no good purpose to multiply instances; there is no opening, the wall is continuous, and is traversed by the tubes of the solenocytes. But there is other evidence of a different nature leading to the same conclusion. I have observed in a living nepliridium the fluid inside the nephridhil canal so compressed, perhaps by the overlying cover-glass, that it dilated the tip of the diverticulum so as to give rise to a bulgiug vesicle at its extremity. Now, such a swelling could obviously not be formed if the tip were open. We may now turn to injectious to corroborate our view. I have receutly injected the dorsal hyperbranchial coslom with Indian ink. The minute black particles were held in suspension in sea-water. Such a fluid, if introduced with a hypodermic syringe, can be made to fill the coslom. It is clear that if the nephridium communicated with the coslom the ink would penetrate into the canal; this would happen
STRUCTURE OF THE EXCRETORY ORGANS OP AHPHIOXUS. 191 all the more easily, since a powerful ciliary current works towards the external pore. Sections of such injected specimens show conclusively that not a single particle of ink has entered the nephridial canal, although the iuk lias penetrated into every chink of the ccelom. But, it may be asked, if the facts are so plain and conclusive, how is it that so keen-sighted and accurate an observer as Boveri has been deceived?- Well, if it will not be considered presumptuous on my part, I will attempt to explain how the mistake arose. 1 To begin with, the sections he examined were not appropriately stained. The uuclei are clear, but the cytoplasm scarcely staiued at all. In the majority of the sections which I had the opportunity of seeing the wall which closes the tips of the diverticula was very, difficult to make out, though I could detect it on close examination in a suitable light. I naturally turned with great interest to the section given on PI. 33, fig. 17, of the original memoir (1), and of which a photograph is published in the 'Anatomischen Anzeiger' (la). Anyone on first looking at this section might be led to believe in the existence of a funnel. The appearance is extraordinarily deceptive. But it is deceptive, and the deception is due to two things. First of all the nuclei are deeply stained, but the cytoplasm practically colourless and transparent; in the second place the section is thick. The figure given by Boveri is really an optical section of the preparation. The closingwall can, indeed, be seen, but only with the greatest difficulty. The misleading appearance of a funnel is due to the sudden cessation of the nuclei round the base of the solenocyte tubes; an appearance which is further heightened by the limit of the coelomic epithelium at the same spot (see p. 193, and text-figure). Let it not be thought that in insisting on the absence of an opening I am unduly influenced by a, priori considerations 1 Soon after the publication of his paper (la) I wrote to Professor Boveri, who tlien very kindly sent rue his preparations, and I gladly take this opportunity of thanking him for his courtesy.
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On the Structure of the Excretory Organs of Amphioxus. Part 2.—The ...

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