CRANGON VULGARIS. 123 



these "polkeni" arise the inner "keimzellen." Besides this, 

 a gastrulation is described which is peculiar in being at 

 right angles to the longer axis of the egg and to the nor- 

 mal primitive streak. Its history is not traced, but one can 

 hardly resist the impression that this structure has nothing 

 to do with gastrulation, but is merely afolding of the ven- 

 tral surface of the egg, Certainly the figures will support 

 such a conclusion. 



Dr. Ayers ('84) describes the early history of CEcan- 

 thus and differs from all other authors as to his Interpre- 

 tation of the primitive layers. Aecording to him the 

 blastoderm is to be regarded as largely entodermic, the 

 ectoderm at first forming but "a small area on the dorsal 

 side in the region of the gastrula mouth," which gradually 

 "encloses the yolk and endoderm by a genuine epibole." 

 As I understand his description, he regards the germinal 

 area and the amnion as the ectoderm, while the serosa is 

 entoderm, the yolk being "an inert mass of food substance 

 between the particles of which numerous indifferent cells 

 are found." Hence the line between the amnion and serosa 

 is to be regarded as the boundary between ecto- and ento- 

 derm. These membranes now fuse so as to form the well- 

 known double envelope about the germinal area and then 

 the second fusion takes place followed by the rupture in 

 the place of fusion, and eversion of the embryo. The se- 

 rosa now contracts and pulls the amnion from the dorsal 

 surface, while its cells gather together and form a yolk 

 sac which at last comes to lie within the body, being last 

 seen at the back of the head. The amnion is distinctly 

 stated to form the dorsal wall of the insect, while " the 

 serosa functions as a yolk sac" and the "so-called dorsal 

 organ is but the remnant of the yolk sac" (p. 261). In 

 various places the serosa is spoken of as an " endodermic 

 sac," while, in figures 36 and 37 on p. 260, the serosa is 



ESSEX INST. BULLETIN, VOL. XVIU. 16 



