PTEROPODA AND GASTROPODA. 571 



yolk ; successive cleavage reduces it to the mulberry state ; and the 

 subdivision proceeds to such a degree of minuteness, that the yolk 

 seems to have returned to its primitive condition : it is now the 

 germ-mass. 



These divisions and subdivisions take place rapidly under the eyes 

 of the observer. By compression the hyaline centres escape from the 

 yolk-segments or germ-cells. A vibratory movement of the germ-mass 

 gives the first intimation of the ciliated epithelium, which, as it is per- 

 fected, effects a rotation of that body : this movement is increased by 

 warmth and checked by cold. There next is seen a thickening of 

 the integument, with a development of two tubercles ; one of these 

 enlarges and remains above the germ-mass, representing the mantle 

 and visceral cavity ; the other elongates, represents the foot or basis 

 of the embryo, grows quickly, and soon acquires a cavity with fluid : 

 then a bud grows from its hinder part, which also becomes a vesicle. 

 This " caudal vesicle " grows to one-third the size of the rest of the 

 embryo, and contracts so as to drive its contents into the cavity of 

 the embryo, which, in turn contracting, sends the fluid back to the 

 caudal vesicle. Sometimes the caudal vesicle contracts twice before 

 the embryo-sac reciprocates. The liquid so oscillating contains 

 spherical vesicles like blood-discs. " Trabeculae " next appear in the 

 caudal vesicle, which seems a mere appendix to the embryo. The 

 cilia upon the caudal vesicle and tentacles are very small compared 

 with those of the Aplysia. 



With the caudal appendage appears the fissure indicative of the 

 anterior border of the foot ; and in advance of this, a lobe which soon 

 becomes bifid, and forms two tentacles. Above the foot the sub- 

 cBsophageal ganglion is developed. A crystal of carbonate of lime 

 is the first rudiment of the shell, and is deposited in the substance 

 of the skin, above the visceral cavity. The cephalic lobe proceeds to 

 divide into the upper or ocular pair, the second pair of tentacles, and 

 a lower pair of processes forming the sides of the mouth. 



The embryo- sac now becomes pyriform, and is pressed between the 

 advancing head and the shield ; it next elongates and enters the 

 body ; the intestine begins to appear, and the germ-cells may be 

 distinctly seen to pass to and fro into it, by a movement like that 

 which took place between the embryo-sac and the caudal vesicle. 



The third period of development is marked by the appearance of 

 the heart, by the disappearance of the caudal vesicle, by the com- 

 pletion of the digestive canal, and by the total intus-susception of 

 the unconverted germ-mass. 



The rudimentary shell or shield is pushed backwards : the heart is 

 plainly indicated by its pulsations under the shield, before its contour 



