IV PHYLUM CCELENTERATA 2]? 



The oigans are so arranged that in adjacent canals those of the 

 same sex face one another. It will be seen that the reproductive 

 products have, as in Scyphozoa and Actinozoa, the position of 

 endoderm-cells : whether they are developed, in the first instance, 

 from that layer is uncertain. When ripe, the ova and sperms are 

 discharged into the canals, make their way to the infundibulum, 

 thence to the stomodseum, and finally escape by the mouth. Im- 

 pregnation takes place in the water. 



Development. — The process of development has been traced 

 in several genera closely allied to Hormiphora, so that there is 

 every reason to believe that, in all essential particulars, the 

 following description will apply to that genus. 



The egg (Fig. 163) consists of an outer layer of protoplasm (plsm.) 

 containing the nucleus (mi.), and of an internal mass of a frothy 

 or vacuolated nature (yk) : the h/^m 

 vacuoles contain a homo- 

 geneous substance which 

 serves as a store of nutri- 

 ment to the growing embryo, 

 and apparently corresponds 

 with the yolk which we shall ^ 

 find to occur in a large pro- 

 portion of animal eggs. En- 

 closing the egg is a thin 

 vitelline membrane {v.m.), sepa- 

 rated from the protoplasm by ,,„ ,,,., n < t „.-, i 



^ 1- . y I'Ki. 103.— Ovum of Iiampetia. nit. nucleus; 



a considerable space, filled with Plu'"- protoplasm ; r. ,«. vitoUinu mombrano ; 



, . ,, r ' v/i-. yolk. (After Chun ) 



a clear jelly. 



After impregnation the oosperm segments, but the details of 

 the process are very different from those we are familiar with in 

 the other Ccelenterata. The protoplasmic layer accumulates on 

 the side which will become dorsal, and the oosperm divides along 

 a vertical plane, forming two cells each with a sort of protoplasmic 

 cap (Fig. 164, A, plsm.). A second division takes place at right 

 angles to the first, producing a four-celled stage (B), and each of 

 the four cells divides again into daughter-cells of unequal size, the 

 result being an eight-celled embryo, each cell with a protoplasmic 

 cap at its dorsal end (C, D). Next a horizontal division takes 

 place, dividing off the protoplasmic caps as distinct cells, and so 

 producing a sixteen-celled stage (E, F) in which we can dis- 

 tinguish eight large, ventral, yolk-containing cells or megameres 

 {niff.), and eight small, dorsal, protoplasmic cells or micro- 

 mars {mi.). 



The micromeres increase rapidly in number by division, and are 

 further added to by new, small cells being budded off from the 

 megameres (Fig. 164, G, H, and Fig. 165, A). The result of this 

 increase is that the micromeres gradually overspread the megameres 



