25O ERNEST E. JUST. 



But there are other eggs in which the future median plane does 

 not fall in the plane of the first cleavage. In Xereis (Wilson, 

 '92) the second cleavage plane, although it does not divide the 

 animal into "equal halves," coincides with the long axis. So in 

 Crepidjila, the first cleavage plane is at right angles to the future 

 median plane (Conklin, '97). In the newt (Jordan, '93) the 

 case is the same. In Chatopterus (Lillie, '06) the axis of the 

 first cleavage spindle lies in the longitudinal axis of the embryo. 



There is a third group of eggs in which coincidence \vith any 

 cleavage plane is wanting This is true of the egg of Amia (Whit- 

 man and Eycleshymer, '97), of the toadfish (Clapp, '91), and of 

 certain amphibians (Jordan and Eycleshymer, '94), to name a 

 few. And yet in most of these eggs the symmetry and the 

 bilaterality of the cleavage may be sharply marked. 



In the frog's egg the first cleavage plane usually and the median 

 plane of the embryo always (Ran a fused) pass through the en- 

 trance point of the sperm (Roux, '85; Schulze, '99; Brachet). 



In the egg of Toxopneustes (Wilson, '95) the first cleavage plane 

 passes through the entrance-point of the sperm, "in the great 

 majority of cases, at least." This plane of cleavage coincides 

 with the transverse diameter of the embryo (Driesch). 



In the ascidian egg, the belief of Castle ('96) is that the first 

 cleavage plane cuts through the entrance-point of the sperm. 

 Conklin ('05) says that there is no question but that the first 

 cleavage plane is through the copulation path of the germ nuclei. 

 And indeed his figures show very beautifully that this is actually 

 the case. 



If now we grant that in the egg of the frog and of Toxopneustes 

 as in the egg of Nereis and of the ascidian the first cleavage plane is 

 determined by the copulation-path, or the entrance-point, of the 

 sperm we have this interesting conclusion : The first cleavage plane 

 in eggs whose cleavages have different values and different rela- 

 tions to the future long axes of the embryos is determined by the 

 entrance of the sperm. While the sperm entrance determines 

 the first cleavage, the first cleavage does not in all of these forms 

 coincide with the median plane of the future animal. 



Since in the egg of Nereis the sperm may enter at any point and 

 since the first cleavage plane passes through this point, the struc- 



