468 Report of the State Geologist. 



mences to disappear at this stage and already presents vague traces 

 of a welding with the inferior part of the endoderm (fig. 11, m, 

 Plate C), and the cavity of the body as it enlarges laterally begins 

 to recede and disappear at the base of the embryo. Figure 12, 

 Plate C, shows the same modifications much more accentuated. 

 The annular bourrelet is developed in such a manner as to form 

 the most voluminous part of the entire embryo and has quitted 

 its exactly intermediary position, approaching more and more to 

 the aboral pole, to which it is now much nearer than to 

 the oral pole, and the face in which is situated the mouth, 

 from being the smaller, is now much the larger face of the 

 embryo. In this progressive enlargement of the exterior the 

 annular fold which constitutes the bourrelet carries with it the 

 entire layer, which is now strongly separated from the endoderm 

 except at the aboral face, where it is welded with the endoderm. 

 The cavity of the body, previously continuous, is thus reduced to 

 its superior portion, which in turn is divided into two distinct 

 parts,, the part which borders the oral face {cc) and which forms 

 the general cavity, properly speaking, and the part comprised in 

 the bourrelet {cm), which later forms the cavity of the mantle. 

 On the aboral face it is no longer visible except as a scarcely per- 

 ceptible line, which in a confused manner delimits the exoderm 

 from the inferior portion of the endoderm. On all. the rest of 

 this face it is seen that the two primitive layers are thickened in 

 giving birth to a thick white mass of a histological structure difii- 

 cult to determine. The cavity of the body is seen in the middle 

 of this mass very indistinctly, and in the stage represented by 

 fig. 1 3 it has entirely disappeared. 



We have seen that since its first appearance, the median swell- 

 ing and later the bourrelet, at first situated near the oral 

 pole, afterward moves toward the opposite extremity, and 

 finally is situated much nearer to the aboral pole. This process 

 still continuing, the bourrelet is found situated at the extreme 

 limit of the aboral face, necessarily by the continuation of this 

 process, it comes to project beyond the aboral face in the form 

 of a mantle (fig. 13, Plate C). The position of the bourrelet at 

 the extreme limit of the aboral face modifies very much the 

 general aspect of the embryo. It is no longer composed of two 



