Lamprey The egg of the lamprey {Figure 7-1 B) is ovoid 

 in shape and covered by a chorion (shell and microvilli) 

 and a vitelline membrane. There is a small opening, the 

 micropyle, through the shell at the apex of the egg. The egg 

 is essentially isolecithal, although there is an accumulation 

 of larger yolk spheres at the vegetal end. With fertilization, 

 the membranes lift from the egg surface, forming a perivi- 

 telline space (Figure 7-6). The cleavages are as in Amphi- 

 oxus, and the resulting blastomeres are fairly equal in size. 

 A blastula is formed with a large blastocoel. The dorsal wall 

 of the blastula is about two cells thick. Invagination begins 



with the formation of a blastopore groove at what will be 

 the posterior end of the organism. Involution of cells occurs 

 particularly at the dorsal blastopore lip; there is some 

 epiboly, overgrowth by the dorsal lip. 



With the completion of invagination, a medullary plate 

 forms along the dorsal midline. This plate gradually sinks 

 as the outer neural ridges move inward to meet at the mid- 

 line, thus forming a solid neural strand or keel. The blasto- 

 pore remains open throughout the process of neural keel 

 formation. 



As this keel is formed, the mesoderm and notochord 



I Z_sperm 



micropyle / '^ 



icrovilli 



■perivitelline 

 space 



A FERTILIZATION 



B 8 CELL 



CROSS SECTION 



EARLY BLASTULA 



neural keel 



notochord 



neural keel 



a\ keel 



E EARLY GASTRULA 



mesodermal band 



notochord 



blastopore 



F SAGITTAL SECTION G CROSS SECTION 



EARLY NEURULA 



liver diverticulum 



blastopore 

 H EMBRYO OF 3 SOMITES 



notochord 



pineal 



nasal pit 



neural cavity 



I EMBRYO OF 10 SOMITES 



Rathke's pouch 



J EMBRYO OF ABOUT 35 SOMITES 



Figure 7-6. Stages of the lamprey {Lampetra). A, upper end of egg shov/ing micropyle for 

 entrance of sperm and perivitelline space which forms after fertilization; B, 8-cell stage; C, section 

 of early cleavage stage; D, section of early blastula; E, sagittal section of early gastrula, anterior 

 end to left; F, sagittal section of early neurulo; G, cross section of early neurulo; H, sagittal section 

 of embryo with three somites; I, sagittal section of embryo of 10 somites; J, sagittal section 

 of embryo with 35 somites. (A, after Nelsen, 1953; B, C, D after Glaesner; E, F after Selys- 

 Longchamps; H, I, J after Goette) 



198 • EMBRYOGENESIS OF THE CHORDATES 



