ORGANIZATION AND CELL-LINE ACE OF A.SCIDIAN EGG. 47 



the mid-line and right at the middle of the crescent, of which they form an 

 extremely small part. They arc formed by the aggregation at this point of a little 

 clear protoplasm which first appeared at the time of fertilization as a clear area 

 around the spermatozoon, and which afterwards lies at the middle of the crescent 

 (figs. 173, 17->. 17li). In Cynthia this area of clear protoplasm does not usually 

 take the form of the deeply staining hosses or caps before the 1 (i-cell stage (figs. 

 113. 110), though these may sometimes appear, as they do in Ciona, at the 8-cell 

 stage. Although they arise from the surface of the crescent they contain no yellow 

 pigment, and in the living egg this small spot of protoplasm ami the cells to which 

 it gives rise are almost perfectly transparent and are therefore difficult to see. In 

 stained preparations they always stain deeply and thus form an excellent land- 

 mark (figs. 116-120, et seq). 



Chabry and Castle have called particular attention to these prominences of 

 clear protoplasm which are found at the posterior pole of the egg, and Castle traces 

 them hack to the 2-cell stage, and ives good reason for believing that this area 

 of clear protoplasm marks the point of entrance of the spermatozoon, and was 

 caused by it. I entirely agree with Castle that this aggregation of clear protoplasm 

 is caused by the entering spermatozoon, since I have seen it surrounding the sper- 

 matozoon immediately after its entrance (fig. 173); but it can scarcely be said to 

 mark the point of entrance, as it does not remain stationary hut moves with the 

 crescent from a point near the vegetal pole to one near the equator on the posterior 

 side of the erg. So far as I am able to determine from a study of Castle's fieures 



GO i O 



and description, the area of finely granular protoplasm, which he represents in his 

 figures 17, 45, 40 and 47, is the middle portion of the crescent. The large area of 

 clear protoplasm represented in each of these figures and marked x gives rise to 

 the middle cells of the crescent (C 5 - 2 , D 52 of his fig. 49), therefore the small pos- 

 terior mesenchyme cells, C" r ' and D 75 of later stages, can represent but a small part 

 of the area marked x in the earlier stages. The earliest stage in which Castle 

 represents the substance of these future mesenchyme cells is at the posterior pole 

 of the cells C 63 and D 63 in his figure 51. 1 conclude, therefore, that he observed 

 the middle portion of the crescent (x of his figures) in the earliest stages of the 

 development, but that be did not recognize the substance of the small mesenchyme 

 cells as distinguished from the substance x (crescent substance) before the 24-cell 

 stage (his fig. 51). 



All students of ascidian embryology agree that the first plane of cleavage is 

 median in position, the second transverse, and the third horizontal or coronal, but 

 beyond this there are few agreements among them, as has been pointed out. In 

 the matter of the relations of these cleavage planes to the germ layers there are as 

 many opinions as there are concerning the orientation of the v^<j:. Van Beneden 

 and Julin (1884) maintained that the four ventral cells of the 8-cell stage are 

 purely ectodermal, but that the four dorsal cells are still mixed." each of them 

 containing ectoderm and endoderm, while not until the 44-cell stage is the separa- 

 tion of ectoderm and endoderm in these dorsal cells completed. Seeliger ( 1 885) held 



