308 



NEW YORK STATE MUSEUM 



Fig. 



9Polygyra albolabris, showing the generative 

 organs in situ : 1 genital opening ; 2 penis ; 3 vagina ; ^ 

 vas deferens ; 5 spermatic duct ; 6 uterine canal ; 7 talon ; 

 8 albumen gland; 9 hermaphroditic duct; 10 ovotestes; 

 11 salivary gland ; 12 stomach ; 13 rectum ; IJf beginning 

 of rectractor muscles ; 15 buccal body ; 16 tentacular 

 muscles. 



PLATE 24 

 Pig. 



1 A group of eggs, natural size. 

 2-5 Various forms of eggs, x 4. 



6, 7 Transverse sections of round and oval eggs. 



8 The germinative vesicle, x 48. 



9 The under side of the germinative vesicle at the beginning 



of segmentation, x 48. 



10, 11 The ovum during segmentation as seen from above, x 48. 



12 A side view of the same, x 48. 



13 The segmented mass as seen from below, showing blasto- 



pore, endoderm and ectoderm, x 48. 



14 A vertical section of the same, showing 1 the blastopore; 



2 the archenteron; 3 cleavage cavity; 4 ectoderm; 5 endo- 

 derm. 



15 Cells. The dark cells are from the ectoderm; the light 



from the endoderm, x 250. 



16 Showing the endodermal cell mass surrounded by a trans- 



parent sac or wall, x 32. 



17 Showing the early stages of the formation of the foot and 



shell sac, x 32. 



18 A stage slightly more advanced, showing the beginning 



of the tentacles and shell, x 32. 



19 A stage still more advanced showing the beginning of the 



podO'Cyst at the extremity of the foot, x 32. 



20 A view of the same, from below, showing the podocyst and 



the mouth of the animal, x 32. 



