58 



passes at first forwards and downwards, then conies round 

 into the mass of the gland. A great number of smaller 

 lateral ductuleo open into these prhicipal ducts, and on 

 them are arranged in clusters the secreting alveoli of the 

 gland. 



The epithelium of the stomach passes gradually into 

 that of the ductules of the gland. The tricuspid body 

 disappears ; the long spindle cells become shorter, and a 

 corresponding decrease in length of the cilia takes place. 

 But the epithelium of the ductules (fig. 17, PI. III.) always 

 consists of spindle cells carrying cilia, and their walls are 

 thrown into slight longitudinal folds ; at the passage of 

 the lumen of the ductule into that of a secreting alveolus 

 a rapid transition from this ciliated epithelium into that 

 characteristic of the alveoli takes place (fig. 15, PI. III.). 



The fixation and subsequent treatment of the digestive 

 gland, so as to exhibit the nature of the secretory epithe- 

 lium, is difficult, but good results are to be obtained by 

 McMunn's method. A very small piece of the gland is 

 rapidly removed from the living animal, and at once put into 

 a 20% solution of commercial formaldehyde in water. The 

 tissue is allowed to remain in this for about 24 hours, and 

 is then transferred to 70% spirit and dehydrated, embedded 

 and cut in the ordinary way. The sections are stained 

 with Mayer's glychaemalum and eosin, or with Heiden- 

 hain's iron haematoxylin. With fixing reagents of less 

 penetrative power the epithelium breaks up in the process. 



The lumen of the alveolus is always a very restricted one, 

 and is usually cruciform in transverse section. The spindle 

 cells becomes cubical and the cilia disappear (fig. 15, PI. 

 III.). Then the cubical epithelium becomes replaced by 

 four groups of large club-shaped cells (fig. 16, PI. III.). In 

 tangential sections of the alveolus these cells, which are 

 then cut transversely, have very definite polygonal out- 



