408 



PROFESSOR LANKESTER. 



lative ontogeny of a large number of organisms living at 

 this day it forms by rupture. 



Leaving now for the moment the task of further detailing 

 the hypothetical changes which the ancestors of the Entero- 

 zoa underwent (to which we shall return), let us, having thus 

 pictured to ourselves the steps by which a hollow cell-mul- 

 tiple, derived from a single cell, gave rise to an enteric cell- 

 layer by delamination, and acquired a mouth with stomo- 

 dseum and an anus with proctodseum — whilst various changes 

 of general form were aflFected and sundry tentacular and 

 such-like organs probably developed — inquire how the ob- 

 served facts of the early stages of individual development in 

 animals can be explained by applying to these facts and to 

 our hypothetical sketch the doctrine of heredity, viz. that 

 the development of the individual is a recapitulation of the 

 development of the species, interrupted and modified by 

 processes of adaptation. 



Hypothesis of the substitution of Invagination for Delami- 

 nation. — Whilst according to the above hypothetical sketch 

 of ancestral development from the monoplast to the diblastula 

 — the primitive enteron or digestive cavity is the blastocosl and 

 the enteric cell layer forms by the Delamination of its wall — 

 we find in the actual development of animals that the process 



Tic). JO 



Firj.m 



Tigs. 8, 9, 10. Stages of egg-division and invagination without forma- 

 tion of a pseudoblastula. Fig. 11. Pseudoblastula. Fig. 12, 13. Epibolic 

 invagination. Figs. 14, 15. Invagination as seen in Eartliworm and 

 Neniatoda. I^c. Deron ; En. Enteron ; Blp- Blastopore. 



of Invagination in one modification or another, is almost uni- 

 versal. Indeed, Professor Haeckel and Professor Huxley 

 are inclined to think that it is universal. We have, how 



