No. 2.] EARLY DEVELOPMENT OF AMBLYSTOMA. 351 



It often happens that some of the furrows, instead of being 

 horizontal, are more or less inclined, at times passing into the 

 vertical. Jordan and myself ('92) described such variations in 

 the Newt, Frog, and Amblystoma. Morgan and Tsuda ("94) 

 have since recorded similar variations in the Frog's Q.g^. 



Fourth cleavage. — About i hr. and 50 min. later, the fourth 

 set of furrows appear. In the majority of cases they are 

 vertical, as shown in Fig. 8. They often all depart from one or 

 the other of the first two verticals; sometimes, however, some 

 depart from one, while the rest depart from the other, as shown 

 in Fig. 15. In short, this set offers so many variations that 

 a type is no longer recognizable. In the Qg% represented 

 (Fig. 1 5) the first of this set appeared in quadrant b at the first 

 vertical. The next was formed in the adjacent quadrant a 2 min. 

 later, its point of origin being at one of the second verticals. 

 The third furrow, starting from the first vertical, passed across 

 quadrant d i min. later ; the division of c followed i min. later, 

 and this set passed toward the lower pole, which point some 

 reached, while others turned aside and united to form a com- 

 mon furrow. 



The entire set of verticals may depart from a common point at 

 the superior pole and cut each quadrant into two approximately 

 equal portions, in which case a radially symmetrical condition 

 obtains. It was this condition which von Baer ('34) observed 

 in Rana and considered as a type. Yet Rauber ('83) denies 

 this a typical value, and states that he has not seen a single 

 ^ZZ where such was the case. 



Another condition often observed is one in which the posi- 

 tion of the furrows in hemisphere ab (Fig. 1 5) would be similar 

 to that in cd\ in this case there is a distinctly bilateral appear- 

 ance. The furrows often run nearly parallel to the first ver- 

 tical, and again parallel to the second, recalling the condition 

 observed in Teleosts. Such variations in the Frog's Q.gg have 

 been figured by Prevost and Dumas ('24), Max Schultze (63), 

 Rauber ('83), and others. 



Fifth cleavage. — In this cleavage (Fig. 9) the furrows are 

 both horizontal and vertical, the majority being horizontal. If 

 we follow this cleavage in detail (Fig. i6), we find the first fur- 



