270 bulletin: museum of comparative zoology. 



formed the longitudinal musculature. It would seem, therefore, that 

 the muscle plate of the mesodermal somite of Amphioxus is homologous 

 with the muscle cells of the Ciona tadpole. Both in Amphioxus and in 

 Ciona the muscle fundament arises from cells lying lateral to the chorda 

 and derived from the primary ectoderm. In Amphioxus the muscula- 

 ture, like the chorda with which it is intimately associated, becomes 

 (ccenogenetically '?) extended far forward to the anterior end of the 

 trunk region ; whereas in Ciona neither musculature nor chorda extends 

 farther forward than about the middle of the trunk region. 



The mesoderm lateral to the muscle plates of Amphioxus seems to be 

 the homologue of the mesenchyme of Ciona. Both are derived from 

 the endodermal portion of the mesoderm. (Cf. the quotation from Lwoff, 

 pages 268, 269.) 



My conclusions differ from those of Lwoff chiefly regarding the origin 

 of the chorda. He considers this organ to be derived from the primary 

 ectoderm in Amphioxus and the Vertebrates, whereas I regard it as 

 formed in Ciona exclusively by the primary endoderm. I think that 

 Lwoff has been led to include the chorda cells in the primary ectoderm 

 chiefly because they are in Amphioxus (as in Ciona) smaller and clearer 

 than the less rapidly cleaving endoderm cells. These criteria I regard 

 as insufficient. Only a study of the cell lineage can give in any case 

 a positive answer to the question whether the chorda cells at the begin- 

 ning of gastrulation lie in the outer or the inner layer of the embryo. 



That a distinction is rightly made in the case of Ascidians between 

 the two kinds of mesoderm which I have recognized, viz. musculature 

 and mesenchyme, is unanimously agreed to by embryologists ; but the 

 fact has been heretofore overlooked that these two kinds of mesoderm are 

 derived from different fundaments early distinguishable both histologically 

 and topograph i call )/, and that these fundaments "hould be regarded as 

 derived from different primary germ layers. 



A minor point of theoretical importance is whether or not the chorda 

 shall be regarded as a mesodermal organ. Lwoff does not so consider it, 

 though he recognizes two facts which, it seems to me, would naturally lead 

 one to that conclusion : the first, that in Amphioxus and the lower groups 

 of Vertebrates the chorda is derived from a common fundament with 

 what is universally regarded as mesoderm ; the second, that the chorda, 

 like the undoubted mesoderm, comes to occupy a position between the 

 inner and outer layers of the embryo. For these two reasons, which 

 I have shown to exist also in the case of Ascidians, we must, to be con- 

 sistent, regard the chorda as a mesodermal organ. 



