776 



ZOOLOGY 



SECT. 



to the germinal disc. At an early stage in the process of division 

 the blastoderm exhibits a distinct bilateral symmetry. This 

 meroblastic segmentation results in the formation of a nearly 

 circular blastoderm, the outer cells of which tend to separate off. 

 At first the blastoderm consists of only a single layer of cells 

 the ectoderm, which gradually extends. At a later stage a second 

 layer (Fig. 708, B, C) appears below the margin of the blastoderm. 



sh.al 



yk* posljlf 



yh.l 



den, 



a,r" 



FlG. 709. Early stages in the development of Loligo. A, stage at which tlir rudiments <>t 

 the eyes and of the shell-gland are lirst distinguishable ; B, later embryo from the oral 

 .side ; C and D, from the anal side. ant. /. /. anterior funnel fold ; ar. rudiments of arms ; 

 cten. ctenidia ; eye, eye ; mo. mouth : mant. rudiment of mantle ; oto. statucyst ; )>o*t. /. /. 

 posterior funnel fold ; sit. gl. shell-gland ; yk. s. yolk-sac. (After Korschelt and Jlcid.-r. i 



and extends inwards until it comes to underlie the greater part 

 of the embryonic part of the blastoderm : separating this from the 

 yolk is a thin layer of uncertain derivation the yolk-epithelium 

 (Fig. 708, yk. ep.). There is some doubt as to the nature of the 

 second layer ; it certainly gives rise to the mesodermal structures, 

 and by some observers it is also said to form the epithelium of the 



