LATER LAEVAL DEVELOPMENT OF AMPHIOXUS. 221 



respect with the primary paired atrial cavities, or, as they have 

 been called, branchial canals of the Ascidian embryo ; and the 

 final opening to the exterior in both cases is connected with 

 the original left diverticulum. 



The two embryos represented in the woodcuts are thus 

 regarded as practically identical with one another in all mor- 

 phological respects. There is, in fact, no positive difference 

 between them except in the relative proportion of parts of the 

 body ; the general absence of mesoblastic somites in the 

 Ascidian being, as before stated, secondary and correlated 

 with the atrophy of the subchordal portion of the archen- 

 teron. 



It will possibly have been noticed that the forward exten- 

 sion of the notochord in Amphioxus commences at a remark- 

 ably early stage, months before the larva takes to the sand. 

 This can be accounted for as a hastening of the development, 

 a familiar phenomenon. 



The divergence, then, between the two embryonic types 

 consists in the great development of the praechordal region in 

 the Ascidian embryo, and in the reduction of the same region 

 in the embryo of Amphioxus. 



It will have been inferred from what has been said that the 

 tail of the Ascidian tadpole is regarded as the equivalent of the 

 trunk of Amphioxus. 



In the Ascidian the expanded portion of the prsechordal 

 region after giving rise to the peribranchial diverticula, becomes 

 the branchial sac or pharynx, the floor of which is specialised 

 as a glandular organ known as the endostyle. 



The narrow portion of the prsechordal vesicle, which also 

 extends a very short distance beneath the anterior extremity 

 of the notochord, gives rise to the oesophagus and stomach, 

 while the intestine arises as a lateral outgrowth from the latter; 

 but the extraordinary feature of it is that the intestine grows 

 out from the stomach to the right of the mid- ventral line, 

 and then passes across to open into the cloacal vesicle on the 

 left side. 



There can be little doubt that the ancestor of the Ascidians, 



