114 GENERAL CONSIDERATIONS. 



exterior at one or more places. In the latter case it would 

 constitute the first stage in the evolution of the Metazoa. 



There is no reason to suppose that the protoplasm of such a 

 form, even though partially broken up into areas round the 

 various nuclei, would thereby lose the power of taking in to 

 itself foreign substances which were presented as nutri- 

 ment; in other words, would not prevent it from dis- 

 charging the functions discharged by the protoplasm of all 

 Protozoa, and by the parenchyma cells or phagocytoblasts of 

 Metschnikoff. 



To sum up, while fully agreeing with Metschnikoff, that the 

 formation of the endoderm by invagination of the wall of a 

 hollow blastosphere is a secondary process, I cannot accept his 

 position that the hollow blastula is a primitive form, or that the 

 formation of the endoderm by migration inwards of the cells 

 is a primary process. It seems extremely probable that the 

 blastula has arisen to provide for the better nutrition of the 

 growing embryo, and that the inwandering and invagination 

 are alike secondary processes, the object of which is, when the 

 proper stage is reached, to get the protoplasm back to its 

 central position and ready for the development of the system 

 of the channels which render its maintenance in the inside 

 possible. 



(2) The mesoderm in Peripatus arises from certain nuclei in 

 tbe middle ventral line behind the blastopore. These nuclei 

 may, as I have attempted to show above, fairly be regarded as 

 corresponding with the nuclei in the lips of the blastopore, 

 intermediate in character as well as in position, between the 

 ectodermal and endodermal nuclei. The multiplication of these 

 nuclei gives rise to a primitive streak, which, as in the Verte- 

 brata, is entirely posterior to the blastopore, and is marked by 

 a longitudinal groove — the primitive groove. 



This process resembles, in all essential points, the formation 

 of the greater part of the mesoderm in other Tracheata from 

 the walls of the germinal groove, differing only in this, that 

 whereas in the latter the germinal or primitive streak occupies 



