THE MICROSCOPE IN ANIMAL HISTOLOGY. 215 



more highly developed, and the ovum contained in them is 

 also increased in size and enveloped in a distinct mem- 

 brane. There are from twelve to twenty mature follicles 

 in the ovarium, named, from their discoverer, Graafian 

 follicles. Each has an epithelial lining, in which the ovum 

 is imbedded. The capsule of the ovum is known as the 

 zona pellucida, or chorion, and the albuminous cell-body is 

 the vitellus. The nucleus is situated excentrically, and is 

 called the vesicula germinativa, or germinal vesicle of Pur- 

 kinje. "Within it is a round and highly refractive nucle- 

 olus, the macula germinativa, or germinal spot of Wagner. 

 A Graafian vesicle bursts and an ovum is liberated at 

 every menstrual period. During the progress of the latter 

 down the Fallopian tube to the uterus, impregnation may 

 take place by the penetration of spermatozoa into its yelk. 

 Then the inherent vital energies of the cell are aroused, 

 and the process of segmentation begins. Unimpregnated 

 ova are destroyed by solution. The ruptured and emptied 

 Graafian vesicle becomes filled up with cicatricial connec- 

 tive tissue, which constitutes what is called the corpus 

 luteum, after which it gradually disappears. 



II. ORGANS OF ANIMAL LIFE. 



1. Locomotive. The microscopic structure of bone and 

 muscle has been described in connection with elementary 

 tissues. Tendons and fascias belong to the connective 

 tissues. 



2. Sensory. The nervous apparatus of the body, whose 

 histological elements were treated of on a previous page, 

 has been classified physiologically into: 



1. The sympathetic system, consisting of a chain of gan- 

 glia on each side of the vertebral column, with commu- 

 nicating cords or extensions of ganglia, visceral nerves, 

 arterial nerves, and nerves of communication with the 

 cerebral and spinal nerves. The chief structural differ- 



