ORIGIN OF THE GERMINAL LAYERS. 339 



The four most important cases in which the two processes 

 coexist are the Porifera, the Ccelenterata, the Nemertea, ajidjhe 

 Brachiopoda. In the cases of the Porifera and Ccelenterata, 

 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 abbre- 

 viated there is an invaginate gastrula, 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 abbreviation in the develop- 

 ment. 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 invagination ; 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 

 favour of either of these processes by a comparison of the cases 

 where they occur in the same groups. 



The relative frequency of the two processes supplies us with 

 another possible means for deciding between them ; and there is 

 no doubt that here again the scale inclines towards invagination. 

 It must, however, be borne in mind that the frequency of the 

 process of invagination admits of another possible explanation. 

 There is a continual tendency for the processes of development 

 to be abbreviated and simplified, and it is quite possible that the 

 frequent occurrence of invagination is due to the fact of its 

 being, in most cases, the simplest means by which the two- 

 layered condition can be reached. But this argument can have 

 but little weight until it can be shewn in each case that invagi- 

 nation is a simpler process than delamination ; and it is rendered 

 improbable by the cases already mentioned in which delami- 

 nation has been secondarily derived from invagination. 



If it were the case that the blastopore had in all types the 



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