ON THi: (îASTRULATrOxN IN PF-TIIO.M V/.( )N. 43 



segmentation cavity. 1 have eudeavoured to sliuvv hore the 

 transitional part ot' the micromeric to the macronieric laver and 

 the mode in which tiie micromeric layer of many cells in liiick- 

 ness (tiiion.) passes over into the columnar epithelium (inic. ep.). 

 liesides the dilïerence in size of cells the nuclei contained in the 

 micromeres (upper part ol the figure) are ranch smaller than those 

 in the raacromeres (lower part) ; this contrast is, in this section, 

 especially shar{)ly expressed. The lighter colour of the micromeric 

 part is due to the smaller quantity of yolk-granules contained. 

 The e})ithelially differentiated part passes over, on one hand 

 abruptly into the macronieric i)art formed of a crowd of larger cells 

 and, on the other hand, into the micromeric layer which is not 

 yet well established but is formed of cells wedged in between one 

 another in a dovetail-like manner, x ISO. 



Fi(/. 24 — A transverse section throngh the dorsal part of an ovum abont 

 as far developed as that shown in Fig. 10«. The distinctive 

 characteristics of the two kinds of cells are the same as given in 

 the explanation of Fiç/. 23. Between the outer {e.mic.ep.) 

 and the inner {i.mic.ep.) micromeric epithelium no difference in 

 histological nature can be detected. The cells of the inner epithel- 

 ium forming the roof of the archenteron [)roliferate on either side 

 of the layer and give rise to cells of the same kind (g.vi.), and 

 which are destined to establish the gastral part of the mesoderm. 

 The concave surface of the archenteric floor is nothing else than the 

 depression or funnel-groove of the exposed macromeric field, x 180. 



Fig. 25 — A frontal section through the blastopore of an ovum of about the 

 same stage as that represented in Fig. la. A large notch (bp.) 

 at the lower end shows the blastoporic depression on the macro- 

 meric field. The macromeric surface is almost entirely with- 

 drawn, and the lateral lips on either side of the notch are covered 

 with the micromeric epithelium. A shallow depression on either 

 side {h.g.) represents the boundary groove. As the section passes 

 through the dorsal part, the segnientation cavity {s.c.) is com- 

 paratively small ; on the contrary, the disturbed part of the 

 macromeres («s) is met with in its whole breadth. The micro- 

 meric layer (micep.) presents the structure of a columnar epithel- 

 ium throughout its whole extent, x 80. 



Fig. 2C — A section from the ventral part of the same series as that shown 

 in Fig. 25. The micromeric layer is nowhere tlifferentiated at 



