148 PHYLUM C@LENTERA. 
accumulate within the ovum as “yolk spherules” or ‘‘ pseudo-cells.” 
Some yolk-granules, formed within the ovum, may coalesce in ‘‘ pseudo- 
cells” of another type. With increase of size the ovum changes its 
form from amceboid to cake-like, and from that to spherical. Around 
the spherical ovum a gelatinous sheath is formed. When the limit of 
growth is reached, the nucleus or germinal vesicle divides twice, and 
two polar bodies are extruded at the distal pole. There are twelve 
chromosomes to begin with, and by the reduction division in forming the 
first polar body, the number is reduced to six. Thereafter the ectoderm 
of the parent Aydra yields to the increasing strain put upon it, and 
Fic. 70.—Development of Aydra.—After Brauer. 
1. Sf., spermatozoa. 3 
2. Amceboid ovum; g.v., germinal vesicle or nucleus; y.s., yolk 
spherules. 
3. Ovum with lobed envelope (sz.) around it. 
4. Ovum protruding ; ., the nucleus ; ecf., the ruptured ectoderm ; 
end., the endoderm. 
5. Section of blastosphere—Zct., ectoderm;. Znd., endoderm— 
being formed. 
6. Section of larva. Zct., ectoderm; Exd., endoderm; g.c., gut 
cavity: sk., ruptured envelopes. 
ruptures, allowing the ovum to protrude. By abroad base it still remains, 
however, attached to the parent, and in this state it is fertilised, the 
spermatozoon entering by the distal pole (Fig. 70, 4). 
The segmentation which follows is total and equal, and results in 
the formation of a blastosphere (Fig. 70, 5). By inwandering, or by 
division of the cells of the blastosphere, an internal endoderm is formed, 
and this formation takes place on all sides. In a word, it is multipolar. 
The segmentation cavity of the blastosphere is thus filled up, and the 
two layers become differentiated from one another, 
The outer or ectodermic layer forms—(a) an external “ chitinoid ” 
shell of several layers; (4) an internal membrane, homogeneous, thin, 
