239 



There is no general agreement, however, as to the factors which 

 determine the antero-posterior axis of the embryo. It has been held 

 by some that the path of the spermatozoon, together with the long 

 axis of the egg, determines the position of the first cleavage plane 

 and that this plane in turn delimits the right and left halves of the 

 future embryo. While this may be true of the frog it is not true of 

 the newt as Jordan ('95) has shown. Schultze regarded the ex- 

 centric position of the germinal vesicle as the determinant of the first 

 cleavage groove. The later studies of Roux, Jordan and others have 

 shown that this is very questionable. The unequal distribution of 

 pigment has been considered as significant for the orientiation of the 

 embryo by Roux, Morgan and others. The observations of Mos- 

 KOWSKI, Morgan's recent observations on the toad, and my own on 

 Amblystoma lead one to hesitate before accepting the hypothesis that 

 there is a constant relation between pigmented areas and cleavage 

 planes. 



One is thus led to question if any of these factors is of funda- 

 mental importance in the orientation of the embryo, and if not, what 

 is the first visible change which will enable us to predict the antero- 

 posterior axis of the forthcoming embryo. 



One of the most constant, and possibly significant, features in 

 the cleavage of the vertebrate egg, is the early appearance of a se- 

 cond area of accelerated cell-division. This area in Necturus is not 

 as well marked in the egg represented in Fig. 46 as is usually the 

 case. It will be seen, however, that the upper portion is considerably 

 in advance of the portion of the egg which lies toward the bottom 

 of the picture. 



KoELLiKEE ('79) first called attention to this area in the blastodisc 

 of the chick and suggested that it determines the position of the 

 posterior end of the embryo. The later investigations of Duval ('84) 

 and KiONKA ('94) leave no doubt as to the frequent and probably 

 constant appearance of this area in the locality which later becomes 

 the posterior end of the embryo. The studies of Lwoff ('94) show 

 that a like area forms the anläge of the posterior end of the embryo 

 of Amphioxus. The figures of the segmenting blastodiscs of the 

 Elasmobranchs, given by Balfour ('78), Rückert ('85), Gerbe ('92) 

 and SoBOTTA ('98) all show that in these forms such an area is present. 

 In the Amphibia the observations of Morgan and Tsuda, Schultze 

 ('00) and my own ('95), ('98), show that such an area is present in 

 many of these forms. Vay ('93) from his studies on Tropidonotus 



