germinal disc 



central cells 



yolk mass 



marginal blostocone 

 cytoplasm 



2 CELL 



blostodisc 



B 



germinal rm 



prechordal plate 



embryonic shield 

 notochord 

 entoderm 



periblast 



EARLY CLEAVAGE 



blostodisc 



BLASTULA 



^ limit of germinal ring 



prechordol plate fmesentoderm) 



neural keel 



notochord 



entoderm 



KupfFer's vesicle 



periblast 



D BEGINNING OF GASTRULATION 



embryo 



E STAGE 13 TROUT ^ LONGl SECTION STAGE 13 TROUT 



blastopore' 

 germinal rin 



neural keel 



neural keel 



eye 



germinal ring 



periblast 



Kupffer's vesicle 

 yolk plug in blastopore 



periblast 



notochord 



H 



YOLK PLUG STAGE 



I CROSS SECTION OF LATE GASTRULA 



G YOLK PLUG STAGE 



Figure 7-11. Stages of a teleost, mainly the Sea Boss (Serronus atranus). A, section of 

 2-cell stage; B, section of early cleavage; C, section of blastulo; D, sagittal section of gastrula (head 

 to left); E, appearance of blostodisc and early embryo (trout, stage 1 3); F, sogittol section through 

 early embryo (trout, stoge 1 3); G, yolk plug stage; H, details of posterior end of embryonic body 

 at late yolk plug stage; I, cross section through neurula (late yolk plug stage). (After Wilson, Witschi, 

 1956, and Nelsen, 1953) 



mesoderm-free area is lacking and a midline vitelline vein 

 drains the yolk sac. The paired ventral aortae arch upward 

 into the head and give rise to a number of arches. These 

 shorten as the head rises up from the yolk sac. The yolk sac 

 is probably supplied vi'ith venous blood. 



The larva resembles the lamprey in that the entire length 

 of the mid and hind guts is occupied by the yolk mass. The 

 head end develops rapidly and extends far out from the yolk 

 mass. The tail end appears much later as the yolk mass 

 shrinks to the general body dimensions. 



The relationship of yolk to larva and the general shape of 



the egg appear to be the same in both the lamprey and the 

 myxinid, but there is a great difference in their styles 

 of cleavage and the details of development. For example, a 

 neural tube is formed directly by invagination in the 

 myxinid. This type of tubulation is observed in most verte- 

 brates and is quite different from that observed in the lam- 

 prey or the teleost. The style of the teleost must be consid- 

 ered a modification. 



Mammals The eggs of mammals range from a reptile-like 

 form with a leathery shell to a very small isolecithal type 



206 • EMBRYOGENESIS OF THE CHORDATES 



