672 WILLIAM PATTEN. 



pole of the egg, and may be said to lie parallel with its short 

 axis (PL XXXVIb, fig. 5, and PL XXXVIa, fig. 2). During the 

 formation of the embryonic membranes the egg becomes sphe- 

 rical ; and still later, just before the appendages have appeared, 

 it again assumes an oval form, the ventral plate now, however, 

 extending in the direction of the long axis of the egg. 



These changes are brought about by an increase in the length 

 of the ventral plate, and its tendency to lie straight in the egg. 

 This is rendered possible by the flexibility of the chorion, which 

 yields to such an extent as to readily allow the transformation 

 of the short diameter into the long one. When the ventral 

 plate has attained about one half its ultimate length the egg 

 assumes a circular form, owing to the equilibrium existing 

 between the extensive force of the germ band and the tendency 

 of the egg to retain its oval shape. This circular form, how- 

 ever, is retained but a comparatively short time, for in about 

 twenty-four hours the germinal band has so increased in 

 length as to convert what was the short axis into the long one 

 (compare PL XXXVIa, figs. 4, 5, and 6) ; here the egg is seen 

 from the dorsal side, and simply shows the two ends of the 

 germinal band, which has already begun to encroach upon the 

 dorsal surface of the egg. 



During the formation of the embryonic membranes there 

 may be observed a median longitudinal infolding of the ger- 

 minal band (PL XXXYIb, fig. 9), which commences in the 

 posterior end of the embryo and gradually extends towards the 

 head, almost reaching the point where the mouth appears later; 

 in optical section it has the appearance shown in PL XXXVIa, 

 fig. 5. It will be seen in the next stage that this fold entirely 

 disappears (PL XXXVIb, fig. 11), and is not to be confounded 

 with a median fold in the same place that appears later, and 

 which initiates the formation of the nervous system (PL XXXVIb, 

 fig. 12). The first furrow gives' rise to a part of the endoderm 

 and all of the mesoderm, hence we will call it the "gastrula.^^i 



■" Objection has been made to the use of the term " gastrula " in this con- 

 nection, on the ground that the invaginated cells, do not give rise to the endo- 

 derm. This objection has in the present instance been met by showing that 



