VIII 



AETHEOPODA 



259 



cylindrical aad secrete abundant chitin, forming a horny ring from 

 which a valve-like outgrowth projects ventrally {stig, Fig. 199, 0). 



We saw that as development proceeded the embryonic area 

 increased in length, till by the time it is fully segmented it extends 

 round the posterior end of the egg on to the hinder portion of the 

 dorsal surface. This results in what is known as the dorsal 

 curvature. Then the amnion and serosa become adherent to one 

 another in the hinder region of the embryo, and a perforation is 

 effected at this point. There follows a process closely analogous to 

 that described as reversion in the embryo of the spider ; that is to 

 say, the dorsal regions of the embryo grow more quickly than the 

 ventral, and the hinder portion of the embryo is protruded through 

 the hole in the membranes. The adhesion of the two membranes to 

 one another progresses rapidly forwards, and, step by step with 

 it, the tear or rip which exposes the body of the embryo increases 

 in extent. 



The lateral portions of the embryonic area grow rapidly upwards 

 along the yolk towards the mid-dorsal line, and, since the embryonic 

 membranes arise from near the lateral edges of the embryonic area, 

 the lines of origin of these membranes become shifted farther and 

 farther upwards towards the 

 mid-dorsal line. When the 

 embryonic area has so far 

 extended round the sides of 

 the egg that its edges have 

 nearly met in the mid-dorsal 

 line, the remnants of amnion 

 and serosa become in- 

 vaginated into the yolk and 

 form a tube, lying along the 

 dorsal surface of the embryo ; 

 this tube is termed the 

 secondary dorsal organ. 

 The formation of this dorsal 

 organ by the involution of 

 the remnants of the amnion 

 and serosa is progressive ; it 

 is begun behind and gradu- 

 ally travels forwards. As 

 soon as the invagination at 

 any point is complete, dis- 

 solution of the invaginated 

 cells rapidly follows, so that 

 at any one time the dorsal 

 organ is of limited extent. 

 After the completion of this , . , . 



organ the layer of cells of the embryonic area, which are derived from 

 the ectoderm, completely invests the egg. Fig. 205 shows the process 



Fig. 205. — Two diagrammatic sagittal sections 

 through the egg of Periplaneia orientalis in 

 two stages of development. (After Heymons. ) 

 A, before reversion. B, after reversion. a&i-5, the 



abdominal appendages ; cim, amnion ; s.d.o, secondary 



dorsal organ ; ser, serosa. 



