326 Thos. H. Montgomery. 



blast cells. At the stage of the early abdominal segments are found in 

 the abdominal midline separated patches of small cells ( G. C. ?, Figs. 

 54, 56b, 61, PL V) ; these may be definitive entoblast, but differ from 

 the entoblast of later stages in being branched and from the vitello- 

 cytes in their much smaller size, therefore they may be germ cells. 

 In later stages I could not distinguish germ cells from entoblast, and 

 the genital organs arise considerably later than the stage of reversion. 



II. Summary of Observations With Comparison of the 

 Literature. 



11. Cleavage Up to Gastrulation. 



The egg before segmentation show^s cytoplasm around the central 

 pronuclei, and a delicate netw^ork, placed betw^een the yolk globules, 

 connecting this central cytoplasmic mass with a fine peripheral layer. 

 The yolk consists of an outer layer of radial pyramids and an inner 

 layer of large granules not so disposed ; I have not specially studied 

 the segmentation of the yolk during cleavage because the fixation 

 employed coagulated the yolk in the earlier stages. Cleavage consists 

 in repeated nuclear divisions, the nuclei as they become more numer- 

 ous move nearer and nearer to the surface of the egg, vrhereby the 

 central cytoplasmic mass divides into as many portions as there are 

 nuclei and the intravitellar cytoplasmic network shortens until all 

 nuclei and all cytoplasm become placed on the surface. During the 

 earlier cleavage a central fluid mass forms in the egg. Inequality in 

 rate of nuclear division commences at the 32-cell stage. At the 

 stage of 140 cells all the nuclei have become superficial, equally 

 numerous at all points on the surface, thus forming the early blasto- 

 derm. No nuclei remain in the yolk and no polarity of the egg can 

 be distinguished up to this stage. On the blastoderm the ventral 

 embryonic area (germ disc) becomes established by more rapid multi- 

 plication of nuclei in that region and by migration of other nuclei 

 toward that pole; then for the first time appear distinct cell mem- 

 branes, and these form gradually as the separate cytoplasmic masses 

 come in apposition. The superficial cytoplasmic layer is not divided 

 into cell areas before this period. 



In much of the preceding work more attention had been given to 



