376 Formation of the Germinal Layers in Vertebrates. 



animals tlie notocliord is only found in the tail. Wc might 

 imagine such an animal as an Ascidian larva to have been 

 developed in the following manner : — The proliferation arising 

 at the posterior end by multi|)lication of the cells, which 

 represents the chordo-muscular rudiment, instead of growing 

 forwards between tlie intestine and external covering, as in 

 Vertebrates, formed an appendage in which the notochord 

 and the lateral musculature were diflereutiated from the 

 conmion rudiment. It must be pointed out that the lateral 

 muscles are present only on either side of the notochord, i. e. 

 only in the tail of the larva ; in the body of the Ascidian, 

 •where there is no notochord, they are entirely absent. This 

 shows with sufficient clearness that the lateral musculature 

 and the nolochord Iicre by no means arise from the elements 

 of the endoderm (dorsal portion of the intestine) as is 

 commonly supposed, but, as I think, from a special continuous 

 ectodermal rudiment. In Amj^hioxus and the Vertebrates the 

 proliferation of the ectoderm cells proceeds from the hinder 

 margin in such a way that the chordo-muscular rudiment 

 grows forward between the two primary germinal layers, and 

 thus forms a plate of cells which is situated between the 

 nervous system and the intestine. The ingrowth of this 

 rudiment from the posterior margin of the enveloping layer 

 may be regarded as an ancestral feature, since in the case of 

 the Aseidians also the proliferation takes place at the posterior 

 margin. But this process was subsequently capable of being 

 so far modified that in certain Vertebrates, in addition to 

 growth from ihe margin of the enveloping layer, splitting-off 

 of Ihe ectoderm cells is also observable. From this point of 

 view the dorsal wall of the gastrula of AmiyJiioxus, the dorsal 

 wall of the corresponding stages in Petromyzon and the 

 Amphibia, the blastoderm of the Selachians and Teleostean 

 fishes, and the ectodermal portion of the germinal disk with 

 the primitive streak and cephalic process of the Amniota, 

 each lepresent a continuous rudiment, from which the nervous 

 system, the notochord, and the lateral musculature are deve- 

 loped. 



(7) The fact that the notochord, although it arises from an 

 ectoblastogenous rudiment, nevertheless enters into a tran- 

 sient connexion with the cells of the endoderm, may also be 

 turned to account for the purposes of phylogeny. It points 

 to the conclusion that the notochord soon after its origin 

 entered into intimate relations with the intestine, and perhaps 

 also commenced to serve for its attachment. This con- 

 nexion, however, is to be regarded as being of a secondary 

 nature. 



