92 



MANUAL OF ELEMENTARY ZOOLOGY 



in the epidermis or scarf skin which covers the surface 

 of the frog's body. In it the lowest layer consists of 

 deep cells with unaltered protoplasmic bodies, but suc- 

 cessive layers from within outwards become more and 

 more flattened and converted into a horny substance 

 till the outer layer consists of flat, horny scales which 

 are shed, being renewed from within by the division of 

 the lower layer. The inner, softer strata are known as the 

 Malpighian layer. Germinal epithelium consists of columnar 

 or cubical cells with rounded cells derived from them, some 

 of which give rise to ova and spermatozoa. It is found 

 lining the seminiferous tubules of the testes (Fig. 402) 



and covering 

 ,l.c. the surface of the 



^um. ovaries. The sper- 

 matozoa are minute 

 structures consist- 

 ing of an elongated 

 head, which con- 

 tains the nucleus 

 in a very thininvest- 

 ment of cytoplasm, 

 a short neck, which 

 consists of proto- 

 plasm containing 

 a centrosome (see 

 p. 104), and a tail, 

 which has the form 

 of a flagellum or 

 lash of protoplasm. 

 The ova are large, 

 rounded cells con- 

 taining numerous 

 granules of food matter or yolk and blackened on one side 

 by the presence of pigment. On this side lies the nucleus. 

 There is no centrosome. Each ovum is enclosed in a 

 vitelline membrane, and surrounded in the ovary by a case 

 or follicle of cells, derived from the germinal epithelium, 

 which manufacture the yolk and pass it to the ovum. 



Nervous tissue consists of cells provided with processes 

 for the purpose of conducting impulses. In each such 



Fig. 51. — A small portion of a section of a 

 frog's liver, highly magnified. 



6.V., Blood vessel ; l.c., liver cells; /urn., lumen of 

 liver tubes ; r.c. , red corpuscles. 



