CLEAVAGE AND FORMATION OF THE GERM-LAYERS. 



109 



to the Turbellaria, but the mode of formation of the mesoderm described by 

 this author so little agrees with what is found in other Gastropods, that it 

 must be regarded as quite improbable, especially when we remember that 

 BLOCHMANN, who investigated the ontogeny of Aplysia at the same time as 

 MANFREDI, saw nothing of this process, and MAZZARELLI who, quite recently, 

 has made similar investigations, describes the formation of the mesoderm in 

 an entirely different way. 



FIG. 40. A-H, diagrams in illustration of the cleavage and formation of the germ- 

 layers in the Gastropoda (principally after RABL and BLOCHMANN). A and B, seen 

 from the side ; C-F, seen from the animal, and H from the vegetative pole ; G 

 represents an optical section. 7-71' denote the large cleavage-spheres, from which 

 the micromeres (7'-7F', I"-IV"} are abstricted by successive divisions, 1-4, micro- 

 meres, arising from J'-IV. eef, ectoderm ; ent, entoderm ; mes, mesoderm ; rk, 

 polar bodies. 



The rudiments of the germ-layers develop, as in the Amphineura 

 and Lamellibranchia, very early. In Planorbi*, according to KABL, 



