VI. 



TISSUES IN GENERAL. 



ORIGIN, DEFINITION, AND DIVISION OF TISSUES. 



ORIGIN. All complex organisms, the higher developed ani- 

 mals, originate from an ovum of the female impregnated by 

 the admixture of spermatozoids of the male. The ovum, 

 inclosed by a hyaline layer (zona pellucida *of Von Baer), is 

 composed of living matter in reticular arrangement (the germ of 

 Remak), which contains a nucleus-like body, the vesicula germ- 

 inativa, with a varying number of coarser granules, the nucleoli, 

 the macula? germinativae. In mammals and some amphibia, 

 the germ, in toto, is transformed into the animal, whereas in the 

 eggs of birds, scaly amphibia, and osseous fishes, a portion of the 

 erm is changed to yolk, which serves as a pabulum. 



After the spermatozoids have entered the germ viz. : after 

 fructification has taken place its living matter increases rapidly, 

 the vesicula germinativa disappears, and the germ, by a process 

 of division, splits at first into two portions, separated from each 

 other by a light narrow rim, but connected by extremely delicate 

 filaments, which traverse the light rim. Each half of the germ 

 splits into a number of lumps, which, in the same manner as the 

 first half, remain connected ; thus the segmentation of the ovum 

 results in the formation of numerous corpuscles, which by col- 

 lecting in a flat layer represent the germinal disk of Pander, in 



