1^6 WHITMAN. ' [Vol. I. 



This closure advances from before, backwards, following the 

 direction of the bud-development. The closure of the primi- 

 tive blastopore, on the contrary, progresses in the opposite 

 direction, and represents, not a restoration, but a reconstruction. 

 In the embryonic Trochosphere the anterior remnant of the 

 blastopore persists as the mouth, while in the fcetal Trocho- 

 sphere, where the primitive blastopore never comes to develop- 

 ment, the mouth appears to form as a secondary perforation. 



Grant that teloblasts exist in both the larval and the foetal 

 form, and that the conception of them as a trunk-bud is 

 correct, and I see no escape from the above view of the blasto- 

 pore. The larval form has the primitive blastopore with typical 

 relations to the mouth and to the non-metameric portion of the 

 animal ; the foetal form has lost the primitive blastopore, and 

 acquired a secondary one, which may be regarded as a posterior 

 extension of the original opening along an entirely new region 

 — the metameric trunk-region. 



The primitive Gastrula stage is passed long before the estab- 

 lishment of the first metamere ; the secondary Gastrula is the 

 primitive one extended, and so retarded in development that the 

 process of gastrulation is prolonged through the whole forma- 

 tive period of the embryo. 



The occurrence of the Trochosphere, its origin from a typical 

 invaginate Gastrula, the persistent relations of the blastopore to 

 the mouth, and the presence of a teloblastic trunk-bud, all 

 appear to me to support the views developed in the foregoing 

 comparison. 



It is conceivable that the Gastrula from which the Trocho- 

 sphere arises represents not a primitive, but a derived form, 

 which has been much reduced in extent through a retarded 

 development of the trunk. The objections to this view are 

 numerous, and so obvious that they need not be enumerated. 



The comparison above made between the foetal and the larval 

 Trochosphere has important bearings on -the interpretation of 

 the blastopore in higher forms, on the concrescence theory 

 of the formation of the vertebrate embryo, and on some recent 

 theories of the origin of metameric segmentation, — bearings 

 which cannot be considered in extenso within the limits of the 

 present paper. I may say, however, that I see no reason for 

 abandoning the so-called theory of concrescent growth. In all 



