568 WILLIAM PATTEN. 



the first true cells of the embryo, for hitherto we have had a 

 " syncytium/' By mutual pressure the outer faces of the cells 

 become highly convex. The nuclei are large, and each contains 

 a small, sharply-stained nucleolus. When seen from the surface 

 they appear as represented in PI. XXXVIa, fig. 1 ; in section 

 as in PI. XXXVIb, fig. 4. 



The blastoderm at first surrounds the yolk as a single layer 

 of cells of uniform thickness, which soon become long and 

 columnar at one pole of the e^^, and correspondingly thin and 

 epithelial-like at the opposite pole (PI. XXXVIb, fig. 5). 

 The thin portion of the blastoderm becomes the " serosa," 

 and the thickened part is the ventral plate. Zaddach states 

 that the blastoderm ruptures at the dorsal pole, where it becomes 

 so thin. This, however, is a mistake, as a long series of sections 

 at this stage, and even much later, shows the complete con- 

 tinuity of the original blastoderm at all points of the egg. 



Stage III. — The period covered by this stage extends from 

 the time of the formation of the ventral plate up to the comple- 

 tion of the embryonic membranes. It includes the process of 

 the gastrulation, the origin of the endoderm, and the meso- 

 derm, as well as the formation of the amnion and serosa. 



The embryonic membranes are first seen as faint elevations 

 of the " ventral plate,'' which in optical section appear like 

 a thick layer of columnar cells (PI. XXXVIa, figs. 3 and 4). In 

 actual section they appear like simple folds of the blastoderm 

 (PI. XXXVIb, figs. 8 and 9), which are raised on all sides of the 

 embryonic area, and gradually increase in extent until they 

 finally meet nearly over the middle of the ventral plate. Por 

 the sake of convenience in description one may recognise four 

 separate folds — a head fold, tail fold, and two lateral folds — 

 although, as we have already indicated, they form a continuous, 

 somewhat circular, elevation. PI. XXXVIb, fig. 8, represents a 

 longitudinal section through the ventral plate, showing the head 

 fold at am" and the tail fold at am' ; the latter arises first, and 

 grows more rapidly, often covering more than two thirds of the 

 ventral plate before the lateral folds meet in the central line. 

 This condition is not easy to observe in the living e^g, but 



