310 LECTURE XXII. 



ment depend upon the aggregation of the fissiparous cellules around 

 one or around several centres ; whereby, since all subsequent deve- 

 lopment radiates from such centres, one or many embryos may be 

 matured in the same ovum. Thus the ovum of the Tritonia may give 

 issue to seven or eight embryos ; whilst that of the Planorbis and of 

 most other Gasteropods is the seat of development of a single young one. 

 In the Planorbis the single centre of the ovum, or the germinal 

 vesicle with its nucleus, is very evident in the ovarian ovum. The 

 metamorphoses, which lead to the disappearance of the vesicle^ take 

 place in the oviducal pouch called the ' matrix.' The transparent nida- 

 mentum in which the ova are excluded is shield-shaped and striated ; it 

 is not attached to any foreign body but falls to the bottom. After the 

 usual subdivisions of the yolk, a group of less opake cells makes its 

 appearance in a particular part of the periphery of the granular mass ; 

 and an epithelial membrane begins to spread over its surface, from which 

 cilia are soon developed. Their action begins about the third day to 

 affect the surrounding albumen, and afterwards to rotate the yolk 

 itself. The aggregation of stronger and more numei'ous cilia on a 

 particular part of the surface of the yolk indicates the seat of the de- 

 velopment of the respiratory organs. Two groups of extremely 

 minute and compact cells, covered by a thicker epithelium, project 

 from two other parts of the surface, and constitute the rudiments of the 

 head and foot. The centre of the yolk presents the form of larger 

 and less regular globules, which indicate the position of the wide 

 digestive sac. The rudiments of the head and foot are sufficiently 

 obvious on the fifth or sixth day ; the respiratory organs are formed 

 on the sixth or eighth day, according to the warmth of the weather. 

 On the eighth day the characteristic tentacles begin to sprout from 

 the rudimental head. On the tenth day all that part of the vitellus 

 or embryo which is not occupied by the head, the foot, and the 

 breathing organ, is covered by a thin and transparent pellicle, which 

 is the rudiment of the shell. On the eleventh day one of the large 

 central globules of the yolk begins to distinguish itself from the ali- 

 mentary mass by feeble contractions and dilatations, of which about 

 sixty may be counted in a minute : this is the heart. The mouth can 

 now be discerned, and the small eyespecks appear like black granules 

 at the base of the tentacula. On the twelfth day the embryo moves 

 by its own contractions independently of the rotation produced by the 

 cilia. On the thirteenth day acts of deglutition are discernible ; the 

 embryo swallows the remaining albumen, the anus is completed, and 

 the genital organs begin to be formed. On the fourteenth day the 

 young Planorbis ruptures by more violent contractions the chorion, 

 and escapes into the water, protected by its own flexible shell. 



