ORGANIZATION AND CELL-LINEAGE OF A.SCIDIAN EGG. 21 



In Ciona the same type of protoplasmic movement occurs as in t ynthia, 1 >ut 

 with certain minor differences. The peripheral layer is here decidedly thicker at 

 the lower pole than elsewhere, even before the fertilization of the egg; the nuclear 

 plasm or clear protoplasm is also at this stage distributed as a layer over the entire 

 upper hemisphere of the egg (fig. 172). After the entrance of the spermatozoon 

 the protoplasm of both these layers collects at the lower pole. The nuclear plasm 

 and peripheral protoplasm cannot easily be distinguished in living eggs of Ciona, but 

 in fixed and stained material the latter stains more deeply than the former (figs. 

 172. 173). A crescent of peripheral protoplasm is formed here in the same way as 

 in Cynthia (figs. 175, 170). and it occupies the same relative position (figs. 179 

 183). Though Castle did not observe the peripheral layer of protoplasm and its 

 movement to the lower hemisphere it is evident that he recognized at least a part 

 of the crescent. His figures 17 and 45-47 show the middle portion of the cres- 

 cent in the 2-8 cell stages, and he describes this as an area of finely granular pro- 

 toplasm, which is clear in the living egg, and out of which the small posterior 

 mesenchyme cells are formed. According to my observations these cells arise from 

 a small part only of the middle portion of this crescent, while the greater part of 

 the crescent gives rise to the muscle and mesenchyme cells of the tadpole. From 

 his figures, as well as his descriptions, it is evident that he recognized only a small 

 portion of the crescent, viz.. this median area of "clear protoplasm." 



In many eggs of Ciona, if not in all, clear protoplasm, which is composed of 

 large alveoles, surrounds the entering spermatozoon (fig. 173). Later, when the 

 sperm nucleus moves to the posterior pole, this clear area moves with it. and in sec- 

 tions in the median plane (figs. 175, 170) forms a clear triangular area in the middle 

 of the deeply staining crescent. There is here shown a marked differentiation of the 

 substance of the crescent which continues to lie recognizable throughout most of the 

 cleavage. I have not observed this clear median portion of the crescent in the 4-cell 

 stage, but in the S-cell stage and thereafter it is plainly visible as a deeply staining 

 cap of protoplasm on each side of the mid-line. It corresponds in the main to the 

 'clear protoplasm" described by Castle, which, as he discovered, marks the posi- 

 tion of the sperm nucleus at the posterior pole and which ultimately gives rise to 

 the '-small posterior mesenchyme" cells (B 7 ") at the posterior pole of the gastrula. 

 This same clear protoplasm is present in the middle of the crescent in the Cynthia 

 egg, although it is here obscured by the surrounding yellow pigment; in the un- 

 segmented e^/ it forms a layer of transparent protoplasm over the surface of the 

 crescent, and in the cleavage stages of prepared eggs it is visible as two deeply stain- 

 ing caps of protoplasm similar to those in the egg of Ciona ; it ultimately gives rise 

 to the small posterior mesenchyme cells which are formed from the middle of the 

 crescent and which are composed of clear protoplasm in which there is no yellow 

 pigment (fig. 48. inch.). The substance of the crescent is therefore plainly differ- 

 entiated from the first into these two substances, clear and yellow protoplasm, 

 which remain distinct throughout the entire development. 



