420 



THE MAINTENANCE OF SPECIES 



blastopore 



Diagram to show 

 the closure of the 

 blastopore in a frog. 

 Figures I to III are 

 views from the vege- 

 tal pole. The rota- 

 tion so typical of de- 

 veloping amphibian 

 eggs has been started 

 in III and completed 

 in IV. The view in 

 IV is from the poten- 

 tial ventral side of 

 the embryo. (After 

 Jenkinson.) 



Early Differentiation of the Embryo 



It must be borne in mind that the changes out- 

 lined follow a definite pattern and that some of 

 them are going on simultaneously. One of the 

 first changes after gastrulation is a gradual in- 

 crease in the length of the embryo due largely 

 to the rapid cell divisions about the lips of the 

 blastopore, which forms the exterior opening of 

 the archenteric cavity. The result is a gradual 

 fusion by a backward growth of the lips of the 

 blastopore, which thus produces an elongated 

 line, the primitive streak. This is one of the best 

 known embryological landmarks. Anterior to 

 the primitive streak there soon develops, partially 

 produced by a sinking of the ectoderm, two 

 closely associated parallel folds of ectoderm, 

 which extend anteriorly forming the walls of the 

 neural groove. Gradually an anterior-posterior 

 fusion of the walls of the groove produces the 

 central nervous system, a dorsal tubular structure 

 characteristic of the vertebrates. Sheets of meso- 

 derm likewise grow anteriorly and laterally from 

 the region of the primitive streak, soon splitting 

 distally to form the splanchnic and the somatic 

 layers. Meantime beneath this the gut is form- 

 ing and being pinched off from the yolk beneath. 

 In its anterior part, the pharyngeal gill-pouches 

 and later the gill-slits appear, together with out- 

 growths which form the lining of the thyroid and 

 thymus glands. Posterior to this region there 

 soon develops a ventral out-pocketing of the gut, 

 which later forms the lungs in land animals, while 

 still further posteriad lie the forerunners of the 

 liver and pancreas. 



The degree of closure of the gut along the 

 ventral surface of the embryo is largely depend- 

 ent upon the quantity of yolk present in the egg. 

 An egg containing little or a moderate amount of 

 yolk, as in Amphioxus or the frog, respectively, 



