THE FROG: HISTOLOGY, GERM CELLS, DEATH 91 



in the cell when they are discharged. Such a gland 

 is known as a tubular gland. The pancreas is an example 

 of the more complicated class known as racemose glands, 

 in which the tubes are branched and lined with low, 

 cubical epithelium up to their ends, which are dilated 

 and lined with glandular epithelium. The dilations are 

 known as acini and the tubes leading to them as ducts. 

 The liver is more 

 complicated still, 

 the tubes not only 

 branchingbut rejoin- 

 ing to form a mesh- 

 work, whose walls 

 consist of gland 

 cells. Pavement 

 Hum belongs 



_ at. 



(Fig. 5) to the simple 



class, but is very 

 different from any 

 of those we have 

 seen hitherto. In 

 it the cells are flat, 

 and so thin that their 

 surface is raised 

 where the nucleus 

 lies. They are 

 separated by narrow 

 but distinct lines of 

 intercellular sub- 

 stance which stains 

 strbngly with silver 

 nitrate, and the sur- 

 face has then the 

 appearance of being 

 composed of flat 

 tiles, like a pavement, from which circumstance the name 

 of the tissue is derived. The coelom, blood vessels, and 

 lymphatic vessels are lined with this epithelium. 1 Stratified 

 epithelium consists of several layers of cells. It is found 



1 See Figs. 5 (surface view) and 46 (section, showing surface raised 

 by nucleus). 



Fig. 50. — Diagrams of different kinds of 

 glands. — Partly after Lang. 



A , Columnar epithelium containing isolated gland cells 

 or unicellular glands. B, similar epithelium with 

 the gland cells collected into a group so as to form 

 a Jlat multicellular gland. C, a hollow multi- 

 cellular gland of the simple kind. The figure 

 represents a type intermediate between the sac- 

 cular glands of the frog's skin (Fig. 52) and the 

 tubular glands of the frog's stomach (Fig. 49). 

 The latter, however, may be forked, and thus 

 show a transition to D, the compound or race- 

 mose glands. 



al., Alveoli or acini of the racemose gland ; </., ducts ; 

 /., alveolus or fundus of simple gland ; g.c, gland 

 cells. 



