1907. J NATURAL SCIENCES OF PHILADKLPIIIA. 359 



Fig. 28. — Stage in vvliich the biastoilorm li:i.s overgrown ahrxit one-half of 

 the yolk. Priniordiu of the following organs are ,sh(jwn: Cerebral 

 ganglia {€!>.), velum (K.), buccal gangha (Be), i)leural ganglia (PL), 

 pedal gangha (Pd.), otocysts (between ])lc.ural ami piMhil ganglia), 

 intestine (In.), parietal gangUa (ou eadi side of intestine), and .shell 

 gland. 



Fig. 29. — Stage in which about two-thirds of the yolk ha.s been overgrown. 

 In addition to the organ primordia already" named, the foot is also 

 \isible between the two pedal ganglia. 



Fig. 30. — Final stages in the overgrowth of tiie yolk. The blastopore (Bp.) 

 is a relatively small area of yolk at the vegetal pole, still uncovered by 

 tlie bhvstoderm. The secondary macromeres (4A, 4B and 4C) lie in the 

 lips of tlie blastopore, and wliile they contain many nuclei their cell 

 boilies are still undivided. Th(! velum has begun to extend forward 

 around the anterior side of the blastopore, while the buccal ganglia 

 (Be.) lie in its lateral borders. All the organ primordia named above 

 are plainly \'isible, and in addition the abdominal ganglion may be seen 

 on the left of the intestine. 



Plate ^XXVIII. — Entire embryos, from the closure of the olastopore to the 



formation of the larv^a. 

 Fig. 31.— Embryo showing the blastopore very small. Tiis margin of the 



shell gland has extended widely, while the area within this margin is 



covered by the embryonic shell (Sh.). 

 Fig. 32. — Stage showing the closure of the blastopore and the establishment 



of the pre-oral and post-oral velar bands The margin of tlie shell 



gland sliows a deep notch posterior to the intestine, which grows more 



pronounced in later stages. The two cerebral ganglia are approaching 



each other in front of the blastopore. The primordium of the permanent 



kidney (K.) is recognizable. 

 Fig. 33. — Older stage, in which nearly one-half of the embryo is covenid by 



the shell. The cerebral ganglia are united by a commissure in front of 



the mouth. 

 Fig. 34. — A stage in which about two-thirds of the embryo is covered by the 



shell. 

 Fig. 3.5. — Stage showing head vesicle (II.V.), larval heart (L.H.), mantle 



(M.), columellar muscle (Cm.M.), external kidneys (Ex.K.) and tentacles, 



(T.) in addition to the other organ primordia named in the description 



of figs. 28-30. 

 Fig. 36. — r^ater embryo, showing the formation of the m.antle cavity, gill, 



velar lobes and the twisting of the nerve loop. The primordia of all 



important organs are present at this stage. 



