LESSONS IN ZOOLOG Y. 257 



spontaneous development without fertilization. This frequently 

 occurs among insects. Alternation of generations sometimes 

 occurs among animals as with plants. 



The ovum is a cell composed of protoplasm and containing a 

 nucleus or germinal vesicle. In the ripe ovum a portion of the 

 vesicle with some of the protoplasm is forced out of the egg cell 

 forming the polar cells, the remaining portion as the female pro- 

 nucleus fuses with the spermatozoon or male pronucleus and the 

 resulting body is the nucleus of the fertilized ovum. 



After fertilization the ovum divides into a great number of 

 cells, the mass for a time retaining a spherical form; but at 

 length, by invagination, one side is pushed or drawnin so thatthe 

 mass has a flask shape, with an outer layer of cells, ectoderm, 

 and an inner layer called endoderm, and between these layers a 

 third is formed called mesoderm. The skin, the nervous system, 

 and sense organs of the embryo are formed from the ectoderm. 

 The muscular system, the connective tissues, the corpuscles of the 

 lymph and blood, and the vessels carrying the fluids of the body, 

 all arise from the mesoderm. The lining membrane of the digest- 

 ive cavity and of the glands opening into it are derived from the 

 endoderm ; while the urinary and generative organs arise from 

 all three of the layers. The formation of the ovum and its devel- 

 opment is much the same among all classes of animals. 

 L. s. 17 



