463 



At the anterior end of the germ band, a transverse depression, 

 or invagination, occurs, beneath the floor of which there has devel- 

 oped a large cell-mass composed of more or less circular cells having 

 deeply staining nuclei. These cells represent a further development 

 of the cells mentioned as occurring in the tv^o-day stage below the 

 middle plate. The middle plate at this stage, with its accompanying 

 cells beneath, extends back a little farther than in the two-day stage. 



Just a little distance in front of the transverse depression, or 

 invagination, the large cells which cover the anterior half of the 

 dorsal part and extend to the anterior end of the blastodemi give 

 way to a one-celled layer of somewhat flattened loosely-connected 

 cells, which resemble the cells of the germ band in their structure and 

 in the manner in which they stain. This layer, which at this stage 

 extends backward only a very little way, not yet bridging the in- 

 vagination, represents the beginning of the serosa. At the posterior 

 end of the germ band also, there is a slight transverse groove, from 

 the posterior border of which a few cells extend forward over the 

 posterior end of the germ band. These cells, however, are not dif- 

 ferent in character from those just posterior to them, that is, they 

 have not changed their shape so as to form a layer of flattened cells 

 similar to the one extending backward from the anterior end. At 

 both the anterior and posterior ends the lateral edges of the germ 

 band are sinking slightly below the level of the other cells so that 

 both grooves are slightly crescentic, the horns of the anterior one 

 extending backward, and these of the posterior one extending for- 

 ward. 



The inner protoplasmic layer, which lies between the cells and the 

 yolk mass, is very greatly thickened at the anterior end, where the 

 greatest cell growth is taking place. The posterior end of this layer 

 seems to have contracted somewhat, ending bluntly, and leaving 

 a space between it and the posterior end of the blastoderm. 

 Near the dorsal side the group of small cells, mentioned in the 

 description of the two-day stage as lying just inside the posterior 

 cells of the blastoderm, are seen to be applied to this blunt, posterior 

 end of the inner protoplasmic layer, although they still retain a loose 

 connection with the surface cells. The bacteria mentioned above 

 can still be seen in the posterior cells. There are still a very few 

 cells scattered throughout the yolk mass. 



In the sections I have representing the four-day stage there are 

 few further changes of importance. The serosa has grown farther 

 backward over the germ band, extending about half its length. Its 

 anterior attachment has begun to retreat somewhat over the antero- 



