CLEAVAGE AND FORMATION OF THE GERM-LAYERS. 



Ul 



not seem to be constant (Morgan), and in any case it soon disappears 

 again. This also, if proved to be correct, would constitute a certain 

 analogy with a condition described for the Araneae (p. 39). In 



Fig. 64.— Sections through eggs of Pallene in various stages of blastoderm-formation (after 

 Morgan). In D, an invagination (e) appears in the blastoderm, round which cells (probably 

 the tirst mesoderm-cells) are given off. M, blastoderm; </, yolk; dp, yolk-pyramids; e, 

 aperture of the invagination ; eh, external and internal integument ; fix, cleavage-cavity (?) 



them also there is a transition from total to superficial cleavage. 

 There is also a concentration of the blastoderm towards that pole 

 at which later the first indications of the embryo appear (Fig. 61 C). 



