338 ORIGIN OF THE GASTRULA. 



tion of the epiblast and hypoblast is a secondary modification of 

 an invaginate type (vide Vol. II. p. 457). 



The type of some Turbellaria (Stylochopsis ponticus) and that 

 of Nephelis amongst the Discophora is not capable of being 

 reduced to the invaginate type. 



The development of almost all the parasitic groups, i.e. the 

 Trematoda, the Cestoda, the Acanthocephala, and the Lingua- 

 tulida, and also of the Tardigrada, Pycnogonida, and other 

 minor groups, is too imperfectly known to be classed with either 

 the delaminate or invaginate types. 



It will, I think, be conceded on all sides that, if any of the 

 ontogenetic processes by which a gastrula form is reached are 

 repetitions of the process by which a simple two-layered gastrula 

 was actually evolved from a compound Protozoon, these pro- 

 cesses are most probably of the nature either of invagination or 

 of delamination. 



The much disputed questions which have been raised about 

 the gastrula and planula theories, originally put forward by 

 Haeckel and Lankester, resolve themselves then into the simple 

 question, whether any, and if so which, of the ontogenetic 

 processes by which the gastrula is formed are repetitions of the 

 phylogenetic origin of the gastrula. 



It is very difficult to bring forward arguments of a conclusive 

 kind in favour of either of these processes. The fact that 

 delaminate and invaginate gastrulse are in several instances 

 found coexisting in the same group renders it certain that there 

 are not two independent phyla of the Metazoa, derived respec- 

 tively from an invaginate and a delaminate gastrula 1 . 



1 It is not difficult to picture a possible derivation of delamination from invagina- 

 tion ; while a comparison of the formation of the inner layers (mesoblast and hypo- 

 blast) in Ascetla (amongst the Sponges), and in the Echinodermata, shews a very 

 simple way in which it is possible to conceive of a passage of delamination into 

 invagination. In Ascetta the cells, which give rise to the mesoblast and hypoblast, are 

 budded off from the inner wall of the blastosphere, especially at one point ; while in 

 Echinodermata (fig. 199) there is a small invaginated sack which gives rise to the 

 hypoblast, while from the walls of this sack amoeboid cells are budded off which give 

 rise to a large part of the mesoblast. If we suppose the hypoblast cells budded off 

 at one point in Ascetta gradually to form an invaginated sack, while the mesoblast 

 cells continued to be budded off as before, we should pass from the delaminate type of 

 Ascetta to the invaginate type of an Echinoderm. 



