CASTLE : EMBKYOLOGY OF CIONA INTESTINALIS. 267 



Like the cells of the endoderm strand just auterior to them, they prob- 

 ably become wandering cells. 



In tiie posterior portion of the trunk region, where before the closure 

 of the blastopore the endoderm strand broadens out into a plate of four 

 or more cells, the more laterally placed endoderm cells move dorsad 

 at the closure of the blastopore, and meet in the median plane under- 

 neath the chorda. In this way the endoderm of the trunk region is 

 converted into a closed vesicle, pear-shaped and broadest in its anterior 

 portions ; at its posterior end it is overlaid by the chorda and flanked on 

 each side by the mesenchyme. 



VII. DISCUSSION OF SOME THEORETICAL 

 QUESTIONS. 



The facts presented in the foregoing pages have a certain bearing 

 on several questions of general interest. Of these I shall make brief 

 reference to, — 1. The origin of the germ layers of Chordates ; 2. The 

 Coelom theory ; and 3. The ancestry of Chordates. 



A. Origin of the Germ Layers of Chordates. 



According to the generally accepted doctrine of Haeckel, all the higher 

 metazoa are ultimately derived from a simple cup-shaped or sac-like 

 ancestor composed of two cell layers, an inner and an outer, continuous 

 with each other at the margin of the cup or sac. The two cell layers are 

 called the primary germ layers. The outer layer is known as the pri- 

 mary ectoderm ; the inner, as the primary endoderm. Among the Chor- 

 dates, this supposed ancesti'al condition is most nearly realized in ontogeny 

 in the case of Amphioxus. The homologues of its inner and outer germ 

 layers are traced by embryologists through all the groups of the chor- 

 date phylum. A third or middle layer, derived from one or both of the 

 others, makes its appearance between the two primary germ layers in all 

 the higher Metazoa. Whether this middle layer, or mesoderm, is homol- 

 ogous thi'oughout the different groups of Metazoa is one of the most 

 difficult and disputed questions in the whole realm of comparative em., 

 bryology. Into this question I do not propose to go in this paper ; I 

 shall confine my attention to the mesoderm of Chordates. 



It is commonly believed that the mesoderm of Chordates is derived 

 entirely from the inner germ layer, which is accordingly often referred to 

 as mes-endoderm. With this view, however, my observations on Ciona 



