CLEAVAGE \M> FORMATION OP Till-: c; K KM -LAYERS. 



109 



bo the Turbellaria, but tin- 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 

 I'.i.oc mm \\\. who investigated the untogenv of A/ili/sia at the same time as 

 M \NKKi:i)i. saw nothing of this process, and M v/.y. mu.li.i who, quite recently, 

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

 an eut irelj different way. 



a. 



J3. 



Fig. 40.- -I-//, diagrams in illustration of the cleavage and formation of the germ- 

 layers in the Gastropoda ^principally after Rabl and Blochmamn). A and /.', seen 

 from the side; ('-/■'. seen from the animal, and // from the vegetative pole ; G 

 represents an optical section. I- IV denote the large cleavage-spheres, from which 

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

 meres, arising from /'-/I", eet, ectoderm; ent, entoderm; mes, mesoderm; >/.-. 

 polar bodies. 



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

 and Lamellibranchia, very early. In Planorhis, according to Eabl, 



