RELATION OF THE CLEAVAGE-FORMS. 2$! 



form, that of the Bell-gastrula (Arcld-gastrula, Fig. 6). It 

 is evident that the former has gradually originated from the 

 latter, in the course of a long period of time, by the accu- 

 mulation of nutritive-yelk in the centre of the egg. 75 



The fact that we have been thus enabled to retrace all 

 the numerous and multiform phenomena in the germination 

 of different animals to these four type-forms of egg-cleavage 

 and gastrulation, may be regarded as an advance of the 

 widest significance. Of these four type-forms we have been 

 able to declare that one is the original, palingenetic form, 

 and that the other three are kenogenetic forms descended 

 from the first. The unequal, discoidal, and the superficial 

 forms of cleavage have evidently all originated, in conse- 

 quence of secondary adaptation, from the primary, original 

 cleavage ; and we must consider that the most important 

 cause of their origin was the gradual formation of a nutri- 

 tive-yelk, and the distinction, which is always appearing in 

 an earlier stage, between the animal and the vegetative 

 parts of the egg, between the skin-layer and the intestinal 

 layer. The inter-relation of the four cleavage-forms, with 

 regard to the ordinary distinction between total and partial 

 egg-cleavage is as follows : 



I. Palingenetic (1. Original cleavage (Bell- 1 

 gastrula). 



2. Unequal cleavage (Hood- 



cleavage. 



A. Total cleavage (with- 

 out any independent 

 nutritive yelk). 



gastrula). 

 II. Kenogenetic 



(modified by 3. Discoidal cleavage (Disc-^ 

 laptation) gastrula). B. Partial cleavage (with 



an independent nu- 



4. Superficial cleavage (Blad- 1 tritive yelk), 

 der-gastrnla). 



The lowest known intestinal animals (Metazoa), that is to 



