944 AYERS ON THE DEVELOPMENT 



the concave side of the egg this amnion intervenes between the ventral, or outer, surface 

 of the embryo and the serosa, while on the other side the yolk intervenes between the 

 deep, or dorsal surface and the serosa. The amnion and serosa are not united at any 

 point, but may lie in contact. The stomodaeum and proctodaeum extend into the yolk, 

 but end blindly. The embryo is a closed sac, as it has been since the union of the two 

 lateral folds of the amnion. The serosa now fuses with the amnion lying over the region 

 of the forehead, but the two layers remain separated for the remainder of their extent. 



The resulting membrane becomes first very thin and finally ruptures, so that the amniotic 

 cavity communicates freely with the space between the vitelline membrane and the serosa, 

 but does not open into the serosal cavity, neither does the latter open into the vitelline 

 cavity. (Compare Brandt, 8, Calopteryx, Hemiptera parasita.) By the contraction of 

 the internuclear protoplasm of the serosa (this process may possibly be aided by the exer- 

 tions of the embryo itself, since at this time traces of muscular fibres have made their 

 appearance) its cells, which at first form only a single layer, are greatly changed in their 

 mutual relations, and the serosal sac is changed into a bag, the walls of which are 

 greatlv thickened and furnished with an opening at the end where the fusion with the 

 amnion has taken place. This is the earliest stage in which I have observed the external 

 opening of the yolk sac. In the meantime, the embryo has (pi. 20, figs. 8 and 9) partly 

 everted itself through the opening, and the serosa is thickened at the apex of the egg. 

 The shortening of the serosa consequent upon the contraction of its cells is the mechan- 

 ical force which, applied to the inner surface of the embryo through the yolk mass, causes 

 its eversion through the ruptured wall. The upper lip protrudes first and is soon followed 

 by the head, antennae, and maxillae. The embryo now lies curved across the lower blunt end 

 of the egg with one-half of its body uncovered by the amnion ; the other half still lies within 

 that membrane and retains its terminal flexure (pi. 20, fig. 9). The head now moves upward 

 on the opposite, or convex, side of the egg and the embryo again assumes a position parallel 

 to the long axis of the latter. But compared with its first position the embryo lies on 

 the opposite side of the egg and faces in the opposite direction. The abdomen is now 

 straightened out and the proctodaeum projects into the yolk, which has in the meantime 

 partly descended into the cavity between the ventral and dorsal walls of the embryo. 



At this time the ventral portion of the embryo is composed of three layers : ectoderm, 

 mesoderm, and endoderm. (PI. 22, fig. 1.) The ectoderm of the ventral side is highly 

 differentiated. From it have arisen, in addition to the ventral wall of the body and its 

 appendages, the nervous system, and the epithelial lining of the fore and hind gut. It 

 gradually becomes less complex toward the dorsum, where it is an exceedingly thin cellu- 

 lar membrane in the condition of a syncytium. The mesoderm extends as a more or less 

 complete layer over the inner surface of the germinal band into the pleural, and later into 

 the dorsal, region. The endoderm, as a distinct layer, is limited to a sac-like sheet extending 

 over the ventral surface as far posteriorly as the blind end of the proctodaeum, over which 

 it folds and is finally lost in the yolk. Anteriorly it curves around the stomodaeum and 

 is continuous with the yolk-sac, which still projects beyond the body walls. The com- 

 pleted dorsal wall is first formed in the region of the proctodaeum, and from that point 

 the closure gradually extends forward until the wall encloses the constantly decreasing 

 yolk-sac. 



