REVOLUTION OF THE EMBRYO 



541 



place tear or burst open. Through this rent (a), in the same place in which 

 the original invagination-opening was situated, the amniotic cavity again opens, 

 and through the opening thus formed first the head and then the succeeding 

 segments of the primitive band (Fig. 520, B) pass out, and remain there while 

 the head passes on to the anterior pole of the egg on the ventral side, the embryo 

 thus assuming a position like that of other insects. (Kowalevsky.) 



In the parasitic Hemiptera (Pediculina), according to Melnikow, the opening 

 in the membranes near the head remains permanent, and the embryo becomes 

 everted through it, while the yolk, enclosed in the continuous membrane formed 

 by the amnion and serous membrane, forms a yolk-sac on the dorsal surface. 

 The same process occurs in Mallophaga, and also in CEcanthus, as described by 

 Ayers (Fig. 521). Generally as soon as the embryo passes out of the amniotic 

 cavity the latter soon becomes smaller and finally completely disappears. 



B C 



tun.-,. 



FIG. 521. Revolution of the embryo of (Ecanthus (diagrammatic) : rt, fore, I, hind end of egg; 

 am, amnion ; d, dorsal, n, ventral side of egg ; *. primitive streak ; r, dorsal plate (originating by 

 the contraction of the serosa ()). After Ayers, from Korschelt and Heider. 



As the embryo grows, and the sides grow up and the back closes over, the 

 contents of the yolk-sac are soon taken up and absorbed in the intestinal cavity, 

 which communicates with it. 



In Phylloclroinia, according to Wheeler, the process of revolution is "hurried 

 through by the embryo from the beginning of the 16th to the end of the 17th 

 day." Several successive stages are represented in Fig. 522. In the 15th day 

 the embryo still occupies the middle of the ventral surface of the egg. Soon the 

 envelopes (amnion and serosa, as) rupture, an irregular slit being formed, and 

 soon the egg and embryo are as seen in Fig. 522, B, the embryo standing out free 

 from its envelopes on the yolk, and the edges of its dorsal growing walls (6) are 

 distinctly marked. The tail now lies at the caudal end of the egg (Fig. 522, <7). 

 By the 17th day the walls have closed in the median dorsal line, and the em- 

 bryo has grown in length to such an extent as to bring its head to the cephalic 

 pole (Fig. 522, E). 



Korschelt and Heider consider, since the primitive band of the chilopod myrio- 

 pods (Geophilus) is curved in at the middle and sinks into the interior of the 



