PRECOCIOUS DEVELOPMENT IN EARLY EMBRYO. 



n 



embryi 



Bd. xxxix, Taf. in), we note that tlie Chim;e- 

 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 Chima'ra 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 Chimjera ' ' precocious segre- 

 gation" has been developed to a noteworthy 

 degree. In spite of the small size of the 

 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 dcveloiJcd 

 gut outline, definitely arranged the 

 mesoblast, separated ju'actically the 

 sides of the embryo from the blasto- 

 derm, and has s])ecially 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 (c. g., 

 in his plate xii). 



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 ol section of preceding embryo. The section is transverse and passes near 

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

 a-b. Peripheral zone of blaslod elm ; b-c. central region of blastotferm ; 1-5, centers of proliferation of ecto- 

 derm into mestxferm : 6-7. Lower ends of these proliferations in their relation to entoderm ; 8, amittJsis in 

 spongy trabeculee; 9. grouping of mesoderm cells to form vessels; 10 II. centers, large and small, of 

 germmaUyolk in which or near which nuclear elements are dividing amitotically. 



