292 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



The fragment of yolk that Ues in the periphery takes no further part in 

 furnishing nutriment to the embryo; the one- or two- nucleated inner part, 

 however, continues to function. The embryo as it lengthens rolls up 

 (Fig. 228), the amnion stretching, becoming thinner and membranous 

 in appearance, and finally fusing at the point of tangency whereby it is 

 divided into an inner part (i) and an outer part (o), the latter forming an 

 (>nvelope which soon ruptures and disappears. Within the amniotic 

 cavity an occasional paracyte (par) may be seen. Since the embryo in 

 elongating rolls up spirally, a part of the caudal end is hidden by the head 



stom 



procl- 



Fig. 229. — Stylops. At maximum 

 dorsal curvature, {gc) Germ cells. 

 (t. am) Inner section of amnion. 

 {prod) Proctodaeum. {stom) Stomo- 

 daeum. {y) Yolk. 



proct 



Fig. 230. — Stylops. (gc) Germ cells. 

 {i. am.) Inner section of amnion, (proct) 

 Proctodaeum. (stom) Stomodaeum. {th) 

 Thoracic segments. 



lobes. A diagram of a sagittal section at the time of maximum length is 

 shown in Fig. 229. 



In this stage (Fig. 229) stomodaeum, proctodaeum, and three head and 

 three thoracic segments are in evidence, as well as the single mass of germ 

 cells (gc), the latter therefore late in differentiating. By this time only 

 the inner part of the amnion, which forms the temporary dorsal wall of the 

 embryo, remains; the larger outer part (Fig. 228, o) has already ruptured 

 and disintegrated. Division of the amnion into two parts, one forming 

 the temporary dorsal wall of the embryo, and the other an outer envelope, 

 in a somewhat similar fashion has already been described for the lepidop- 

 teran Diacrisia virginica, where, however, it occurs at a much later period. 

 After the rupture of the outer amnion in Stylops, the segmentation of the 

 abdomen and the development of the thoracic appendages take place. 

 The part of the yolk that remained outside the embryo soon disappears. 



Counter rotation, or the unrolling of the embryo, takes place rather 

 gradually. The head rapidly enlarges; the head lobes decrease in size; 

 the abdomen becomes shorter and sturdier (Fig. 230); and the dorsal 

 amniotic membrane shortens (i). The shortening of the body continues 

 until the embryo is no longer in a spiral but lies with the head in contact 



