iv PHYLUM CCELENTERATA 205 



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

 cap (Fig. 151, 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 

 (mg.), and eight small, dorsal, protoplasmic cells or micromeres (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, 151, G, H, and Fig. 152, A). The result of this 

 increase is that the micromes gradually overspread the megameres 

 (Fig. 152, C), the final result being the production of an embryo 

 consisting of a central mass of large yolk-containing cells (ma.), 

 partly surrounded by an epithelium-like layer, incomplete below, 



FIG. 152. Three stages in the development of Ctenophora. ;un. megameres ; mi. 



(From Lang's Coiiiparatisc Anatomy.) 



micromeres. 



of small cells (mi.). This stage corresponds with the gastrula of 

 preceding types, the micromeres forming the ectoderm, the mega- 

 meres the endoderm, and the ventral edge of the ectodermal 

 'investment representing the blastopore. There is, however, no 

 archenteron or gastrula-cavity, and the stage has been produced, 

 not by a process of invagination or tucking-in, but by one of cpiboly 

 or overgrowth. 



The endoderm-cells increase in number, and become much 

 elongated, and arranged obliquely, their long axes radiating, 

 upwards and outwards, from the long axis of the entire embryo 

 (Fig. 153, A). Their lower (ventral) ends then become divided off, 

 forming a number of small cells, which constitute the rudiment 

 of a true middle cell-layer or mesoderm (A, me.). A kind of in- 

 vagination of the megameres with their mesoderm cells then takes 

 place, resulting in the formation of a cavity the infundibulum 

 (B, d.) bounded below by the megameres, now placed horizontally, 

 and above by the mesoderm. The mesoderm gradually retreats to 

 the dorsal surface (C), finally spreading out between the dorsal 

 ectoderm and the infundibulum. At the same time the ectoderm 

 cells bounding the aperture of the infundibulum grow into it so 



