280 ORIGIN OF TEE GASTRULA. 



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

 genetic 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 processes are most pro- 

 bably 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 gastrulai are in several instances found coexisting in the 

 same group renders it certain that there are not two independent 

 phyla of the Metazoa, derived respectively from an invaginate and a 

 delaminate gastrula^ 



The four most important cases in which the two processes coexist 

 are the Porifera, the Coelenterata, the Nemertea, and the Brachio- 

 poda. In the cases of the Porifera and Coelenterata, there do not 

 appear to me to be any means of deciding which of these processes is 

 derived from the other; but in the Nemertea and the Brachiopoda 

 the case is different. In all the types of Nemertea in which the 

 development is relatively not abbreviated there is an invaginate gas- 

 trula, while in the types with a greatly abbreviated development there 

 is a delaminate gastrula. It would seem to follow from this that a 

 delaminate gastrula has here been a secondary result of an abbrevia- 

 tion in the development. In the Brachiopoda, again, the majority of 

 types develop by a process of invagination, while Thecidium appears 

 to develop by delamination ; here also the delaminate type would 

 appear to be secondarily derived from the invaginate. 



If these considerations are justified, delamination must be in some 

 instances secondarily derived from invagination ; and this fact is so 

 far an argument in favour of the more primitive nature of invagina- 

 tion ; though it by no means follows that in the invaginate process 

 the steps by which the Metazoa were derived from the Protozoa are 

 preserved. 



It does not, therefore, seem possible to decide conclusively in 



/ ^ 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 Ascetta (amongst the Sponges), and in the Echinodermata, shews a very simple 

 way in which it is ijossible 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 Echino- 

 dermata (fig. 199) there is a small iuvaginated sack which gives rise to the hypoblast, 

 while from the walls of this sack amcEboid 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 iuvaginated sack, while the mesoblast cells con- 



I tinned to be budded off as before, we should pass from the delaminate type of Ascetta 



> to the invaginate type of an Echinoderm. 



