290 NEW YORK STATE MUSEUM 



EMBEYOLOGY OF LIMAX MAXIMUM 



When the ova have reached a certain stage of maturity they 

 leave the ovary. This process is known as ovulation. They pass 

 through the hermaphroditic duct (pi. 16, fig. 1, 10), to the albumen 

 gland (9), and thence to the oviduct (7) ; from which they are dis- 

 charged, that is, laid. 



The composition of the egg is as follows : in the interior of the 

 egg is the germinative vesicle, or egg nucleus, which contains a 

 still smaller vesicle, or germinative dot; surrounding these is the 

 yolk, which is a liquid of varying consistency. In the snail it is 

 transparent. These parts of the egg are formed in the ovary. 

 The albumen surrounds the yolk, and in the case of L i m a x is 

 semitransparent, less so than the yolk. This substance is not 

 formed in the ovary, but is secreted by the albumen gland (pi. 16, 

 fig. 1, 9). The albumen is surrounded by a membrane, which in 

 L i m a X is eemi-transparent, so that, when the egg is viewed 

 under the microscope by transmitted light, the movements of the 

 embryo can be observed, though not distinctly enough for study. 



In the eggs of the P o 1 y g y r a the outer covering is still 

 membranous, though much thicker than in Li max, and it is 

 opaque. After the escape of the young P o 1 y g y r a, it has very 

 much the appearance of a calcareous shell. 



Soon after fertilization the egg undergoes the process of seg- 

 mentation, that is breaks up into cells. It first divides into four 

 equal cells; and then smaller cells are formed from the division of 

 the first four, so as to lie outside of them. The smaller cells now 

 subdivide and spread over the first four. The cell mass is dilated, 

 becoming hollow. The large cells subdivide and sink into the 

 hollow of the sphere, forming an elongated groove, the origin of 

 which at the surface is known as the blastopore. This orifice 

 subsequently closes up. The invaginated cells, formed by the 

 subdivision of the large cells, are known as the endoderm. The 

 outer layer, formed by the division of the smaller cells, is known 

 as the ectoderm. This condition of the egg forms the gastrula 

 stage (pi. 24, fig. 10-14). 



A dilatation of the ectodermal walls now takes place, a con- 



