No. I.] GERM-LAYERS IN CLEPS/NE. 175 



is incomplete, and its development is retarded in the interest of 

 the Trochosphere proper ; while in the fcetal form, the trocho- 

 spheral development is abbreviated for the sake of a more per- 

 fect bud with accelerated development. 



Among the more important differences remaining to be noticed 

 are those which have been brought about under the influence 

 of the food-yolk. The process of gastrulation, the form of the 

 blastopore and its relations to the mouth, have been very pro- 

 foundly modified in this way. The trunk-bud of the foetal 

 Trochosphere has been carried far from its original, post-oral 

 position ; and, as the result of this displacement, we see the 

 halves (germ-bands) of the trunk, which develop side by side 

 as a unit in the larva, formed separately, and carried over the 

 massive sphere of yolk in such a manner as to meet along the 

 median ventral line. This whole process of circumcrescence 

 and concrescence has arisen secondarily, in adaptation to foetal 

 conditions that do not exist in the larval form. The blastopore, 

 if we include the space traversed in the closure of the germ- 

 bands, has been stretched out of all proportion to its original 

 dimensions, so that it no longer represents the primitive Gas- 

 trula-mouth, but merely a secondary prolongation of it back- 

 wards along the whole ventral line of the body. In the embry- 

 onic Trochosphere we find the blastopore already closed before 

 the trunk-bud begins to develop; hence the line of closure 

 (" Gastrula-raphe ") is limited to the ventral line of the Trocho- 

 sphere. As the metameric body-region is not yet developed, it 

 is evident that the posterior limit of the primitive blastopore 

 falls within the non-metameric region, from which the head- 

 segment of the adult animal is formed. This region is repre- 

 sented in Clepsine by the cephalic lobe, and the primitive blas- 

 topore is not, strictly speaking, represented at all. The most 

 that can be said is that its position is sometimes indicated by a 

 linear depression (No. i, p. 55), extending from the mouth to 

 the hind edge of the cephalic lobe. The line of junction of the 

 germ-bands becomes continuous with the post-oral groove ; but 

 the two things have nothing further in common, and they are 

 as distinct in meaning as in mode of origin. What is usually 

 called the blastopore is therefore a secondary opening resulting 

 from the mechanical separation of the halves of the trunk-bud, 

 and its closure is simply a restoration of original conditions. 



