472 



make a study of the development of the external form of the em- 

 bryo of that species. 



The eggs were killed and fixed in the same manner as has been 

 described for the eggs of Camponotus. The eggs of Myrmica are 

 much smaller than those of Camponotus, their greatest diameter be- 

 ing about .45 mm. and their shortest diameter being about .35 mm. 

 They may be described as broadly ovate in form, with one end slightly 

 smaller than the other. As in the eggs of Cmiiponotiis, there are 

 two external membranes, the chorion and the vitelline membrane, the 

 chorion being composed of two layers — ectochorion and endochorion. 



The appearance of the first differentiation of the germ band from 

 the undifferentiated blastoderm may be described as follows. At 

 the anterior pole occur two somewhat oval-shaped thickenings of the 

 blastoderm lying side by side wnth a clear area between them. These 

 thickenings are slightly raised above the surrounding blastoderm. The 

 edges facing each other are nearly straight, sometimes concave, while 

 the outer edges are convex. At the opposite pole is a more or less 

 circular denser area surrounded by a clear ring. This clear ring is 

 connected by a light streak with the clear space between the two 

 thickenings at the anterior pole. On each side of this light streak, a 

 slight thickening of the blastoderm indicates the beginning of the 

 germ band. The two oval thickenings at the anterior pole are the 

 procephalic lobes. 



Figure 26, Plate VI, represents an early stage in which the em- 

 bryo is curved around the yolk mass in the form of a capital C. 

 There are slight indications of segmentation. The anterior end is 

 thicker and wider than the posterior end, indicating the earlier de- 

 velopment of the head region. The anterior and posterior ends are 

 seen to be connected by the undifferentiated blastoderm. 



Figure 27, Plate VII, represents a later stage, in which the body 

 segments are clearly indicated. The anterior and posterior ends, 

 both of which are thickened, the anterior much more than the pos- 

 terior, more closely approach each other, and the layer connecting the 

 two ends has a large knob-like thickening near its middle. This 

 thickening is composed of a large irregular mass of cells, and seems 

 to be caused by a crowding and pushing outward of the cells of the 

 layer connecting the two ends, as these ends approach each other. 

 This layer continues at the sides, covering the yolk, and connects 

 ventrally with the edges of the germ band. From a direct dorsal 

 view this thickening is seen to be made up of two parts, separated 

 laterally. This would naturally be the case if the thickening were 

 caused in the way that has been suggested, that is, by the coming 



