1914.] NATURAL SCIENCES OF PHILADELPHIA. 539 



considerable portion of the trunk as well. But the important fact 

 remains that in Halomenia the position of the stomodseum, which 

 marks the position of the blastopore, is unmistakable and, as the 

 diagrams show, it is located immediately behind the border of the 

 test on the ventral surface. Furthermore, the diagrams illustrate 

 the fact that there remain many other exposed cells bounded by the 

 test, and these become directly transformed into the trunk ectoderm. 

 In other words, diagram C is a gastrula stage just as certainly as 

 diagram A, the main differences in the solenogastres being correlated 

 with an epibolic type of gastrulation and the enormous size of the 

 velum. The accurate details of the process are lacking and close 

 comparisons are not possible at present, but the important fact is 

 certainly clear that the cells enclosed by the test are not all endoderm 

 and the blastopore is small and typically situated. 



In later stages certain developmental processes are described that 

 rest in part upon the assumption that all of the region bounded by 

 the test represents the blastopore. In this depressed area the cells 

 are stated to form, by a species of delamination, the future definitive 

 ectoderm and endoderm. The outer layer, circumscribed by the 

 test, now represents the trunk ectoderm, and in it three invaginations 

 soon appear. One of these remains open and becomes the procto- 

 dseum, while the other two soon close and are transformed into 

 mesoblastic bands. Still later the borders of the proctodeum 

 (evidently the large terminal cells of the trunk that form a ring as in 

 D) are said to develop into a sort of caudal button (bouton caudal) 

 that at first projects into the blastocele. Finally the button becomes 

 evaginated and with the trunk ectoderm protrudes beyond the 

 borders of the test. 



In commenting upon these observations it is to be noted that a 

 depression exists in Halomenia within the terminal ring-like group 

 of large cells, but it is in no way connected with the endoderm. 

 No sign of a proctodseum is evident at this time nor has it put in an 

 appearance in a stage considerably beyond the one represented in 

 diagram D. The caudal button is evidently the group of cells that 

 in Halomenia develops from the cells enclosed by the large cells of 

 the terminal ring. As already noted, these at first project into the 

 blastocele, then flatten out, and exposed to the surface are connected 

 with the ventral ganglionic cords. The mesoblast bands are evi- 

 dently these same cords, as will appear more clearly in connection 

 with the cerebral ganglia. 



In the anterior half of the embryo three invaginations now appear 



