germinal area of cytoplasm 



b lasted isc 



blastoderm blastocoel 



peri b lost •'sr"^-': 



•^blastomere mJ 



D blastocone (incomplete cell) 



'oIk nuclei 



blast 



stopore 



blastopore 



epiblost segmentation cavity 



periblast and hypoblast 



hypoblast (mesentoderm) 

 epiblost 



mesoderm free area 



limit of mesoderm 



^^ hypoblast (mesentoderm) 



prechordal plate (mesentoderm) 



otocho 



blastopore 



archenteron ^periblast overlying yolk 



prechordal plate (mesentoderm) 



inner ring sinus (arterial) 



periblast entoderm 



neural groove , 



^ 1 ectoderm 



notochord ^/^' /pVN ,somite 



lateral plate 



vitelline blastopore ^ C^ C ^ ^ ^O ^^ '^ ? -" rOc <^ C^ n'~c ^^ c' ^ 



/ " yolk 



^morginal sinus (venous) 



Figure 7-10. Stoges of the shark. A, first cleavage groove; B, early cleavage of Torpedo; C, early 

 cleavage of Torpedo in section; D, blostulo of Torpedo; E, blastoderm of Scyllium undergoing gas- 

 trulotion; F, sagittal section of blastoderm shown in E (along line a); G, cross section of blastoderm 

 seen in E (along line b); H, early neurula or primitive streak stage of Squo/us (stage 13); I, sagittal 

 section of primitive streok stage; J, sagittal section of neurula of 1 or 1 1 somites of Squa/us); K, 

 cross section of J in region of somites; L, toil bud stage (stage 1 8). (After Ziegler, Rijckert; Witschi, 

 1956, and Nelsen, 1953) 



204 • EMBRYOGENESIS OF THE CHORDATES 



