570 LECTURE xxn. 



from which cilia are soon developed. Their action begins about the 

 third day to affect the surrounding albumen, and afterwards to rotate 

 the embryo itself The embryo now elongates : its hinder end assumes 

 a spiral form. The aggregation of stronger and more numerous cilia 

 on a particular part of the surface of the yolk indicates the seat of 

 the development 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 suffi- 

 ciently obvious on the fifth or sixth day ; the acoustic capsules ap- 

 pear : 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 rudi mental head. On 

 the tenth day all that spirally disposed part of the 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 alimentary mass by feeble con- 

 tractions 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 ten- 

 tacula. 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 re- 

 maining 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. 



The course of development is different in the naked Pulmonata. 

 In the month of February Van Beneden and Windischmann* found 

 ova of a slug (Limax agrestis) buried in groups. They are laid in 

 autumn and remain until spring, resisting the cold that kills the 

 adults. At the beginning of spring the germ and embryo may be 

 found in all stages of development. The egg, when laid, has an ex- 

 ternal lamellated nidamental membrane ; a thin chorion inclosing the 

 albumen; a vitelline membrane with a twisted filamentary appendage; 

 and the vitellus, which is small as compared to the albumen. 



Two clear germ-cells rise from the centre of the vitellus and pass 

 to its periphery : their existence is followed by the first fission of 



* CCCLIV. 



