EMBRYOLOGY. 99 



pact mass several cells thick, some of which are vacuolated, and 

 having a coarsely granular nucleus. Figure 3 of Plate IV shows a 

 cross section — slightly oblique, however — of the germ band. 



The yolk granules of the primary yolk are now more numerous 

 near the embryo. 



In Stage 5 (PL IV, fig. 4) the germ band has clearly differentiated 

 into the amnion and the embryo proper; these gradually merge into 

 each other. Tins differentiation apparently takes place by a gradual 

 migration of cells to one side of the germ band. The cells of the 

 amnion at this time resemble very closely those of the germ band 

 proper. The germ band begins to fold in this stage and its anterior 

 extremity begins to broaden and flatten. The ovarian yolk has de- 

 creased in volume and has assumed a more anterior position in rela- 

 tion to the embryo. The yolk cells in both the primary and ovarian 

 yolk have lost somewhat their amoeboid character, and now consist, 

 each, of a large granular nucleus, with a much thinner area of pro- 

 toplasm about it. The primary yolk granules are smaller and much 

 less numerous than before and are collecting in masses about the 

 yolk cells, with indications here and there of a partition, or wall, 

 forming between them. This stage is reached by the end of the second 

 day, under favorable weather conditions. 



The "polar organ" and protoplasm at the posterior pole contain a 

 large central vacuolar area now. 



In Stage 6 (PL V, fig. 1) the germ band has greatly increased in 

 length, is folded upon itself, and almost forms a loop, the anterior 

 and posterior extremities nearly touching, and both pointing to the 

 posterior pole. A portion of the posterior extremity of the germ band 

 is again folded upon itself. It is now differentiated into three layers, 

 which we take to be, respectively, ectoderm, mesoderm, and ento- 

 derm. The ectoderm and mesoderm consist of a compact mass of 

 columnar cells, two cells thick. The entoderm is much thinner and 

 less compact and forms an almost continuous sheet over the inner sur- 

 face of the germ band. Its cells resemble yolk cells very closely. 



In this stage fundaments of the body segments appear as slight 

 elevations of the ectodermal surface. The ovarian yolk has assumed 

 a more anterior position in relation to the embryo than in the pre- 

 ceding stage. Between the ovarian yolk mass and the germ band is a 

 group of cells that have apparently separated off from the mesoderm. 

 From this group of cells, in later stages, the generative organs arise. 

 The amnion now covers the ventral surface of the embryo and the 

 other surface of the embryo is in contact with the yolk. The amnion 

 is a very thin, delicate membrane, its cells being widely spaced and 

 quite small. The intervening protoplasm between the cells of the 

 serosa has become more constricted and the cells have taken more of 

 an elongated oval shape. The primary yolk has now become defi- 



