308 NEW YORK STATE MUSEUM 



Pig. 

 9 P 1 J g y r a a 1 b o 1 a b r i s, showing the generative 

 organs in situ : 1 genital opening ; 2 penis ; S vagina ; Jf 

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

 8 albumen gland; 9 hermaphroditic duct; iO ovotestes; 

 . 11 ealivarv gland ; 12 stomach ; IS rectum ; IJf beginning 

 of rectractor muscles; 15 buccal body; 16 tentacular 

 muscles. 



PLATE 24 



Fig. 



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, shoeing blasto- 



pore, endoderm and ectoderm, x 48. 



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



2 the archenteron; 3 cleavage cavity; If 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 



podocyst 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. 



