THE ORIGIN OF THE EMBRYONIC MEMBRANES. 5 



The method observed by Kowalevskt and Schulgin must, tbereforej be 

 regarded as the more primitive. Somewhat similar processes are, also, to be 

 found in the Insecta (especially in the Hymenoptera), in which the outer layer 

 of the fold grows out beyond the inner, leaving the latter behind, so that it 

 appears vestigial or else altogether disappears (cf. the account of these | 

 which is given below in connection with the Hymenoptera, the Aphidctt 

 and Oecanthus). In such cases the embryonic envelope consists solely of the 

 outer membrane, the serosa. Reduction does not, apparently, go as far as this 

 in the Scorpiones, two membranes being always found in them (Metschnikoff, 

 Ganin,* Blochmann). Metschnikoff also found that the amnion forms 

 later than the serosa, and this seems to confirm Laurie's view, although 



a. 



Fig. 3. — Eusccrpitis italicus. A, transverse section through tlie posterior part of a g.-rm 

 B transverse section through one of the posterior segments of a germ-disc (with limbs 

 already beginning to form) (after Laurie), am, amnion ; rf, yolk ; ds, yolk-cells ; e, primi- 

 tive thickening, point of cell-proliferation; ect, ectoderm ; ent, entoderm; mes, mesoderm ; 

 n, rudiment of the chain of ganglia ; se, serosa. 



Metschnikoff was not aide to make any exact statements as to the method of 

 development of the inner membrane. While the serosa is composed of I 

 cells, the amnion consists of much smaller cells. Fine filaments, arising from 

 the small cells of the inner membrane (Fig. 5), are said to extend from one 

 membrane to the other. 



The mesoderm cells which, according to Kowai.f.vsky and SCHULGIN, extend 

 between the two membranes of the embryonic envelope, recall the particli 

 yolk which occasionally occur between the amnion and the serosa in the 

 Insecta, a phenomenon which is explained by the method of origin of these 



* Ganin's Russian treatise on the ontogeny of the Scorpion (No. 18, 1867 • 

 which, as far as we know, is not illustrated, we W( ible to examine. 



