197 



the interior, and that the hypoblastic invagination is the resultant 

 expression of this activity. 



Thus the ordinary movements of the cells in effecting throughout 

 the life of an organism, the perpetual and necessary processes of in- 

 gestion may be made to account for the ultimate invagination of a 

 hypoblastic tissue. The inwandering of single cells is replaced phylo- 

 genetically by the immigration of a whole tissue. All agree that the 

 ontogenetic processes are, with few reservations, immensely hastened 

 or accelerated epitomae of the phylogenetic processes. Thus, if the 

 blastula be taken to represent the monoblastic larva, unless the in- 

 gestive functions are active, no inwandering of single cells will take 

 place, but all are peripheral: A process of resultant immigration then 

 takes place to form the gastrula. In this form again the ingestive acti- 

 vity is in abeyance, or there is no inwandering of single cells ^^. If an 

 accurate repetition of phylogeny were shewn in ontogeny we would 

 expect single cells of the blastula to lead the way in migrating in- 

 wards, and later the whole area of ingestion. The gastrulation of the 

 Echinoderma ^^ to some extent, follows this plan. The larval stages 

 of such a form as Amphioxus (blastula and gastrula) are thus not iden- 

 tically similar to ancestral forms, but only to such forms with ingestive 

 migratory cells in abeyance. 



The theory here put forward regarding the origin of the diplo- 

 blastic form from the monoblastic, and the relationship of hypoblastic 

 invagination, has to recommend it several important points : 



1) The actual mechanism of invagination is accounted for by an 

 ingestive migratory force which occurs troughout life in lower forms. 



2) The stages in invagination follow in the sequence of all evo- 

 lution, from single-cell or monocytic immigration to polycytic or tis- 

 sue-in vagination. 



The same arguments may be applied to the history of evolution 

 of mesoblastic pouches from the hypoblast. After the establishment 

 of the hypoblast this tissue becomes with regard to monocytic ingestion 

 the outer limiting layer from wich the inwandering takes place. The 

 invagination of hypoblastic tissue to form a mesoblastic pouch is thus 

 the accelerated resultant of the inwandering activities of the single 



ic It will be seen that, by this theory, the hydroids must be regarded as of a 

 specialised type with reduced mesogloea in accordance an extinction of the process 

 of immigration of ingestive cells. In the Medusae the immigration is present: »The 

 mesogloea is occupied by in-wandering amoeboid cells derived from the endoderm 

 .... The wandering endodermal cells are nutrient in function and represent so far 

 isolated elements of the enteric canal system«. E. R. Lankester, Encyc. Brit. 

 »Hydrozoa«. 



" E. Selenka, Zeitschr. f. wiss. Zool. Vol. XXVII, XXXIII, etc. 



