CLEAVAGE AND FORMATION OF THE GERM-LAYERS. 



141 



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 



35. 



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

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

 the first mesoderm-cells) are given off. bl, blastoderm ; d, yolk ; dp, yolk-pyramids ; e, 

 aperture of the invagination ; eh, external and internal integument ; fh, 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. 64 C). 



