12 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



from Green Turtle Cay, Bahamas. What may perhaps be the same layer was de- 

 scribed bj' Kowalevsky (1866) as a "gelatinous layer," in which the test cells are at 

 first imbedded ; later, about the time of the first cleavage, the test cells move to the 

 periphery of this layer. In these respects this gelatinous layer of Kowalevsky re- 

 sembles the peripheral layer of protoplasm, but in other respects the differences 

 arc very great; for example, Kowalevsky says that this layer is formed by the 

 activity of the follicle cells, that it closely surrounds the blastomeres during cleav- 

 age, and that it becomes the gelatinous mantle of the adult, while the cells within 

 it (test cells) form the cells of the mantle. In none of these respects is this gela- 

 tinous layer like the peripheral layer of protoplasm described above, and I am in 

 doubt whether Kowalevsky actually saw this layer of protoplasm or whether he is 

 not describing the perivitelline space between the surface of the egg and the chorion. 

 It is interesting to note that Kowalevsky calls especial attention to the yellow color 

 of the test cells, though he nowhere indicates that he has seen any such yellow 

 pigment in the egg itself. Kupffer (1870), on the other hand, concluded that the 

 test cells were formed by free cell formation from the substance of the egg, because 

 " die Zellen gleich Anfangs genau die Farbe des Dotters haben' '; however, he 

 nowhere indicates that the peripheral layer of the egg differs in any way from the 

 remainder. In not one of the many later papers on the ascidian egg can I find anv 

 reference to this peripheral layer of protoplasm, except in that of Morgan already 

 referred to. In view of the ease with which it can be seen, both in living and in 

 stained material, and of the very important part which it takes in development, 

 this is most remarkable and inexplicable. If due attention had been given to this 

 feature of the ascidian egg, it is safe to say that some of the most conflicting accounts 

 of ascidian embryology would never have been written. 



A peripheral layer of protoplasm, entirely similar in structure to that of the 

 ascidian egg, has been observed and described by Sobotta (1897) in the egg of Am- 

 phioxus. In this case Sobotta says that the peripheral layer gives rise in large part 

 to the inner egg membrane, which forms about the time of the maturation, but even 

 after the formation of this membrane a portion of this layer may remain at the 

 periphery of the egg. Sobotta speaks of the desirability of observing this layer in 

 the living egg, and from what I have seen in the ascidians I can but emphasize this 

 suggestion. In the ascidians this layer does not disappear with the formation of the 

 egg membrane, to which it contributes, but collects at the lower pole when the 

 egg is fertilized; from Sobotta's figures I judge that the same thing happens in 

 Amphioxus. 



The colors of ascidian eggs deserve some notice at this place. In 1870 

 v. Kupffer observed in the living eggs of Ascidia canina {Ciona inlestinalis f) that 

 in the early stages of the ovarian egg the ooplasm is yellow ; later, as the egg ripens, 

 this color changes to a brownish red (Kupffer, 1870, p. 10). In the later stages of 

 development this red color is limited entirely to the walls of the alimentary tract. 

 In different animals and at different periods of the year Kupffer found that this 

 color varied from a bright red to an orange tone (p. 17). 



