CASTLE : EMBRYOLOGY OF CIONA INTESTINALIS. 225 



The section seen in Figure 15 shows that cleavage has progressed more 

 rapidly from the ventral than from the dorsal surface. This is to be 

 explained by the richer supply of protoplasm on the ventral surface. 



A study by reconstruction or otherwise of a series of sections through 

 an egg in this stage invariably shows that cleavage has also progressed 

 with unequal rapidity from the two ends of the embryo. 



That end at which cleavage is more advanced is destined to become 

 the posterior end. In this case also the inequality in rate of cleavage 

 is attended (probably caused) by an inequality in the distribution of 

 protoplasm. The protoplasmic cap of the ventral hemisphere is always 

 thicker at the future posterior end of the embryo than at the anterior 

 end, and as the first cleavage plane cuts the egg, this accumulation of 

 protoplasm migrates in between the two blastomeres, its presence prob- 

 ably being the accelerating force in the separation of the blastomeres. 



After the first cleavage is completed, the protoplasm, which had mi- 

 grated in between the blastomeres, again returns to the surfiice and takes 

 up a very definite position on the adjacent faces of the blastomeres just 

 below the equator of the egg. (See Plate III. Fig. 17, x.) 



This region appears in the living egg as a clear area, and marks the 

 spot where arise later the small flattened posterior cells found so useful 

 in orientation by Van Beneden et Julin and others. That this clear 

 area is the region of their formation I have been able to establish by 

 continuous observations of the living egg, controlled and completely sup- 

 ported by tiie study of preparations. The thickened spot in the protoplas- 

 mic cap of the ventral hemisphere at the beginning of cleavage, which seems 

 to determine the posterior end of the embryo, I believe to be caused by the 

 entrance of the spermatozoon. It is evident that the spermatazoon, 

 unless it enters exactly at the ventral pole of the vertical axis, must lie 

 upon entrance nearer to one end of the egg than to the other, supposing 

 that it is in the median plane and ventral hemisphere of the embryo. 



The nearer end, I believe, becomes the posterior end of the embryo, 

 and is determined for that fate by the accumulation of protoplasm in 

 the region of entrance of the spermatazoon. It is impossible to say in 

 any particular case exactly where the spermatazoon has entered the egg, 

 fur its presence there cannot be detected until it has begun to form a 

 yolk-free area in the egg. However, I have never observed a case in 

 which the spermatozoon did not give evidence from its position of having 

 entered the egg excentrically with reference to the lower pole of the ver- 

 tical axis. Hence I conclude that cases of entrance at that pole, if they 

 occur, are extremely rare. 



