animal pole 



sperm entrance 



penetration path animal pole 



limit of involution 



gray crescent 

 (dashed outline) 



dorsal blastopore lip 



vegetal pole 

 ventral lip of blastopore 



EGG 



4 CELL 

 evenly pigmented archenteron 



8 CELL 



completion bridge 

 ectoderm 



mesentoderm 



yolk plug 



16 CELL 



neurol groove 



blostocoel 



yolk plug 

 in blastopore 



SECTION OF EARLY BLASTULA 



(SAGITTAL SECTION) 

 YOLK PLUG STAGE 



NEURULA 



midbrain 



neural groove | | jg folding , , . pinea 



neural plate notochord i » I P°^°^^°''^ 



•^ ' ' noTocnora neural crest _,^ 1 A-o^ . 



T-^ — , ,^,^^ yv i^**-T^ , ' ^k\ mesoderm 



/ ^ ^^^^ blastopore 



entoderi 

 mesoderm 

 mesoderm w^q,. diverticulum 



optic vesicle 

 stomodeum 

 solid Rathke's pouch 



oral sucker 



hind brain 



mesenchyme subchordal rod 



neurenteric canal 



(SAGITTAL SECTION) (CROSS SECTION) 



NEURULA 



blastopore and posterior neuropore 



NEURAL TUBE STAGE 



Figure 7-7. Stages of the frog. Outline of egg with a variety of information superimposed; 4-cell 

 stage as seen in dorsolateral view; 8-cell stoge (note irregularity of cleavage planes separating animal 

 pole cells from those below); 1 6-cell stage (note irregular arrangement of animal pole cells); section 

 through the blastulo; external appearance of yolk plug stage (late gastrula); yolk plug stage 

 in sagittal section, anterior end to left; external appeoronce of neurulo stage; sagittal section 

 of neurulo, anterior end to left; cross section of neurulo; sagittal section embryo |ust before hatching 

 (early tail bud). (Mainly after Rugh, 1951, and Huettner, 1949) 



Mesoderm formation involves a sorting of cells at the 

 blastopore. This sorting becomes apparent with the separa- 

 tion of layers some distance from the blastopore. The bilat- 

 eral plates of mesoderm hollovv' out to form the coelom. 



At the completion of gastrulation, the blastopore has been 

 moved nearly halfway around the spherical embryo as a 

 result of epiboly by the dorsal lip of the vegetal half of the 

 original blastula — the animal pole now lies ventroanteriorly. 



A neural plate is formed along the dorsal midline, which 



sinks down between neural ridges. These ridges move up 

 and medially to meet, first near the head end. Closure pro- 

 ceeds anteriorly and posteriorly from this point, producing 

 anterior and posterior neuropores. The posterior neuro- 

 pores lie just above the closing blastopore. As these pores 

 close, the neural canal is continuous with the archenteron 

 by means of the neurenteric canal, which soon closes. A 

 hindgut evagination develops from the archenteron grow- 

 ing downward and backward to meet an invagination 



200 • EMBRYOGENESIS OF THE CHORDATES 



