4l8 MAINE) AGRICULTURAI, ICXPERIMEINT STATION, 1912. 



pens that the muscular layers appear very much as in the trans- 

 verse sections. 



The histological structure of the isthmus proper is practically 

 the same as that of the albumen region. Both sets of glands 

 are present and have practically the same microscopic appear- 

 ance as in the albumen portion. Figure 470 is drawn from a 

 section in the middle of the isthmus. It will be noted that in 

 the epithelium both the ciliated cells and the unicellular glands 

 are present. These have the same appearance as in the albu- 

 men region in a similar stage of physiological activity. 



The tubular glands do not form so thick a tissue layer here 

 as in the albumen portion but in their microscopic appearance 

 they are very similar in the two regions. From figure 470 we 

 note the presence of the dark irregular nuclei lying near the 

 basal walls of the cells. The cytoplasm in this section presents 

 a somewhat more alveolar appearance than is shown in the 

 figures from the albumen region. This is not a constant dif- 

 ference. Either the granular or alveolar appearance may be 

 found in either region depending perhaps on the state of phy- 

 siological activity and also possibly upon the method of fixation. 



Although not shown in the figures it may be stated that the 

 muscular layers, particularly the circular muscles, are somewhat 

 thicker and better developed in the isthmus than in the albumen 

 region. Corresponding to this the walls are firmer and more 

 constricted than in the all^umen region. This is no doubt asso- 

 ciated with the shorter length of time required for the egg to 

 traverse the isthmus. 



It will be remembered that the distinctive function of the 

 isthmus is to secrete the shell membrane (Membrana testacea). 

 We have seen, however, that this region presents no visible 

 dififerentiation which does not also occur in the albumen secret- 

 ing portion of the duct. This similarity in structure does not 

 necessarily imply a similarity in physiological activity. It is 

 quite conceivable that the glands in the isthmus secrete diiTer- 

 ent substances from the similar glands in the upper portion of 

 the oviduct. This question will be discussed more in detail 

 after we have examined the structure of the uterus and vagina. 



