354 DEVELOPMENT OF ORGANS. 



of the mesenteron is at first formed of the three vitelline 

 spheres, from the surface of which a true hypoblastic layer 

 enclosing a central yolk mass becomes differentiated, as already 

 described. The mesenteric sack so formed is constricted by the 

 growth of the mesoblastic septa into a series of lobes, while the 

 posterior part forms a narrow and at first very short tube open- 

 ing by the anus. 



The lobed region forms the sacculated stomach of the adult. 

 The sacculations of the stomach by their mode of origin neces- 

 sarily correspond with the segments. In the adult however the 

 anterior lobe is really double and has two divisions for the two 

 segments it fills, while the posterior lobe, which, as is well 

 known, extends backwards parallel with the rectum, is composed 

 of five segmental sacculations. In connection with the stomo- 

 dasum a protrusible pharynx is developed. 



In Hirudo and Nephelis the mesenteron has from the first a 

 sack-like form. The cells which compose the sack give rise to a 

 secondary deposit of food-yolk. The further changes are prac- 

 tically the same as in Clepsine. In Hirudo the posterior saccu- 

 lation of the stomach is primitively unpaired. The jaws are 

 formed at about the same time as the eyes as protuberances on 

 the wall of the oral cavity. 



BIBLIOGRAPHY. 



(359) O. Biitschli. " Entwicklungsgeschichtliche Beitrage (Nephelis)." Zeit. 

 f. miss. Zool. Vol. xxix. 1877. 



(360) E. Grube. Untersnchitngen iib. d. Entwicklitng d. Anndiden. Konigs- 

 berg, 1844. 



(361) C. K. Hoffmann. " Zur Entwicklungsgeschichte d. Clepsineen." Nie- 

 derldnd. Archiv f. Zool. Vol. iv. 1877. 



(362) R. Leuckart. Die menschlicheu Parasitcn (Hirudo], Vol. i. p. 686, 

 et seq. 



(363) H. Rathke. Beit. z. Entivickhingsgesch. d. Hirndineen. Leipzig, 1862. 



(364) Ch. Robin. Mem. snr L- Dh>eloppement embryogcniqite des Hirndmees. 

 Paris, 1875. 



(365) C. O. Whitman. "Embryology of Clepsine." Quart. J. of Micro. 

 Science, Vol. xvm. 1878. 



\}'ide also C. Semper (No. 355) and Kowalevsky (No. 342) for isolated observations.] 



