CCELENTERATA. 



173 



freely into a stomach with six mesenteries. In the skin are numerous thread- 

 cells. A mesotrochal worm-like larva, also provided with thread-cells, and 

 found at the same time, was conjectured by Semper to be a younger form of 

 this larva. 



Ctenophora. The ovum of the Ctenophora is formed of an 

 outer granular protoplasmic layer and an inner spongy mass with 

 fatty spherules. It is enveloped in a delicate vesicle, the diame- 

 ter of which is very much greater than that of the contained 

 ovum. This vesicle appears to be filled with sea-water, in which 

 the ovum floats. 



Fertilized ova may usually be easily obtained by keeping the 

 captured adults in water from 12 24 hours. The two main 

 authorities on the development of these forms (Kowalevsky, No. 

 147 and 178 and Agassiz, No. 172) are unfortunately at variance 

 on one or two of the most fundamental points. It seems how- 

 ever that the embryonic layers are formed by a kind of epibolic 

 gastrula ; while the true gastric cavity, as distinct from the gas- 

 trovascular, is formed by an invagination, and deserves therefore 

 to be regarded as a form of stomodasum. 



The early stages are very closely similar in all the types so far 

 observed. Segmentation commences by the outer layer of the 

 ovum, which throughout behaves as the active layer, forming a 

 protuberance at one pole, which may be called the formative 

 pole. Close below this protuberance is placed the nucleus. In 

 the median line of the protuberance a furrow appears (fig. 82 A), 



B ^^fiK^, 



FlG. 82. FIVE STAGES IN THE DEVELOPMENT OF IDYIA ROSEOLA. (After Agassiz. ) 



The protoplasmic layer of the ovum is represented in black. 



which gradually deepens till it divides the ovum into two. The 

 granular layer follows the furrow so that each of the fresh seg- 

 ments, like the original ovum is completely invested by a layer 



