VI 



HEXAPODAEMBR YON 1C DEVELOPMENT 



495 



together with this represents an unbroken epithelial membrane, the serous envelope ; 

 this latter surrounds the whole egg with its embryonic rudiment and the amnion on 

 all sides. 



The amnion and the serous envelope have no share in the building up of the 

 embryo. The latter develops exclusively out of the blastoderm of the embryonic 

 rudiment and the invaginated tube, which we will call the germ streak. The blasto- 

 derm of the embryonic rudiment grows further and further dorsally at its peripheral 

 edges, so that at last it envelops the embryo on all sides as ectoderm. Though 

 somewhat out of its strict order, for the sake of clearness, this process as well as the 

 fate of the amnion and serosa may be here illustrated by means of the following dia- 

 gramatic transverse sections. Fig. 351, A, represents the same stage as Fig. 350, F, 



Fin. 351. Formation of the dorsal tube (process of involution of the embryonic integuments) 

 in Hyd.ropb.ilus (after Graber and Kowalevsky). A, Transverse section through an egg, whose em- 

 bryonic rudiment is still covered by the amnion () and serosa (s). B, Amnion and serosa having 

 grown together in the middle line have now torn open, drawing back on each side to form a fnld. 

 C, The fold, by the contraction of the serosa becomes more dorsal. D, The contracted serosa (dorsal 

 plate) is being grown over by the fold. E, The dorsal tube has become closed by the growing 

 together of the folds. F, The mid-gut is closed dorsally and has enclosed the dorsal tube (g). 

 a, Amnion ; d, nutritive'yolk ; h, heart ; ?, ccelome ; m, rudiments of mid-gut ; n, nervous' system ; 

 s, serosa and the structures developed out of it, i.e. the dorsal plate and dorsal tube ; tr, principal 

 tracheal trunk ; ec, ectoderm. 



but the position of the embryonic rudiment is reversed. In Fig. 351, B, we see the 

 amnion and the serous envelope torn in the ventral middle line, after they had 

 previously grown together. The amnion and the serous envelope thus form on each 

 side a fold projecting ventrally. In C the serosa has contracted and has become the 

 so-called dorsal plate, which now consists of high cylindrical epithelium. At the 

 same time the ectoderm of the embryonic rudiments on both sides has extended 



