Teleosts and Birds 



301 



there is established the elongated blastopore 

 more commonly known as the primitive 

 streak. Figvire 111 is intended to illustrate 

 the broad lines of these morphogenetic dif- 

 ferences during the gastrulation period. The 

 striking features here are the peripheral 



longitudinal line and its closure hence en- 

 tirely dissociated from the overgrowth of 

 the yolk by the periphery of the blastoderm. 

 The diagrams of Figure 111.4, taken from 

 the sequence in Fundukis, can serve as a 

 model for teleost development only in a very 



B 





Fig. 111. Diagrams comparing major prospective divisions of the germ in (A) teleost (Fundulus: Oppen- 

 heimer, '37) and (B) chick (Rudnick, '48; Spratt, '52) during gastrulation and later. For obvious reasons 

 most of the yolk mass is omitted in the chick figures. White: ectoderm. Radiating lines in Bl and B2: non- 

 medullary ectoderm. Stippled: superficial material which will later be invaginated. Broken lines: invaginated 

 material. Mesh: extraembryonic ectoderm. Circles: yolk, i, Blastula stage; 2, late gastrula, blastopore nearly 

 closed; 3, embryo formed. 



position of the material to be invaginated, 

 in the teleost blastoderm, as contrasted with 

 its central-posterior position in the bird; con- 

 versely, the central position of the extra- 

 embryonic material in the fish, and the 

 peripheral location of all extraembryonic 

 material in the bird. The blastopore in the 

 first case is the periphery of the disc; in 

 the second case, it is reduced to a central 



general way. Fish eggs vary widely in 

 their proportion of protoplasm to yolk, in 

 the relative time necessary for closure of 

 the blastopore, and hence in the relation 

 of axis formation to invagination. The two 

 best-known forms experimentally, Salmo and 

 Fundulus, are widely separated in the series. 

 Figure 112 diagrams comparable stages as 

 regards blastopore closure in these eggs; 



