PRECOCIOUS DEVELOPMENT IN EARLY EMBRYO. 



77 



Bd. xxxix, Taf. in), we note that the Chimae- 

 roid although smaller is much less flattened 

 out ; that the gut which is flattened against the 

 yolk in the elasmobranch is in Chimaera defi- 

 nitely formed and provided with a distinct 

 lumen; that the lateral contour of the embryo's 

 body in Chimaera is already developed, the 

 ectoderm in the hinder region fusing with the 

 entoderm ; that the yolk entoblast thickens in 

 the median axial line, a feature lacking in the 

 shark, but important doubtless in the early 

 assimilation of the yolk. 



From the foregoing details one is led to 

 conclude that in Chimaera "precocious segre- 

 gation" has been developed to a noteworthy 

 degree. In spite of the small size of the 

 embryo, both relatively and actually, it has 

 already made strides in the direction 

 of attaining its definite form, outstrip- 

 ping in these regards the elasmo- 

 branch ; thus it has already developed 

 gut outline, definitely arranged the 

 mesoblast, separated practically the 

 sides of the embryo from the blasto- 

 derm, and has specially concentrated 

 the yolk entoblast in the axial region. 

 Accordingly, in these regards, Chi- 

 maera stands separate from the elas- 

 mobranchs; transitional, however, is 

 Callorhynchus, judging from figures 

 recently given by Schauinsland (e. g. , 

 in his plate xn). 



An idea of the complicated 

 nature of the blastoderm at this stage 

 (plate v, fig. 35), both in itself and in 

 its relation to the yolk, may be had by 

 examination of fig. 70. This repre- 

 sents part of a section which passes 

 through the blastoderm transversely, 

 somewhat in front of the embryo. 



Fig. 70. Detail of section of preceding embryo. The section is transverse and passes near 

 middle of blastoderm. It shows particularly the early differentiation of the vacuolar area, 

 a-b. Peripheral zone of blastod erm : b-c, central region of blastoderm ; 1-5. centers of proliferation of ecto- 

 derm into mesoderm ; 6-7. Lower ends of these proliferations in their relation to entoderm ; 8. amitosts in 

 spongy trabeculte; 9. grouping of mesoderm cells to form vessels; 10-11. centers, large and small, of 

 germinal -yolk in which or near which nuclear elements are dividing amitotica!ly. 



