THE EMBRYOLOGY OF THE OPOSSUM 51 



During 1 the anoestrum (October, November, December) the 

 uterine mucosa contains many long, tubular, fairly straight, 

 simple, or double-branched glands. The walls of these glands 

 are made up of low columnar or cuboidal cells with large 

 nuclei that nearly fill the cell body. The lumen of the gland 

 is small, and is not apparently ciliated. During the pro- 

 oestrum (first week of January and the beginning of each 

 oestrous cycle thereafter [vide, chapter 1] ) the glands begin 

 to lengthen and coil. The peripheral half of the thickness of 

 the mucosa is the region where the coiling of the glands is 

 most pronounced and conspicuous so much so that the 

 mucosa in sections appears to be made up of two layers. In 

 the medial layer the glands are straighter and are separated 

 by wide lymph spaces; in the distal layer they are so com- 

 plexly coiled that they occupy most of the available space. 

 At this time the nuclei, for the most part, migrate to the bases 

 of the cells, and cilia become abundant in the lumen. During 

 oestrus and ovulation the lumen of the gland enlarges, the 

 number of mitoses increases, and the cilia become more promi- 

 nent. During pregnancy the glands increase progressively in 

 their complexity, even becoming considerably coiled in the 

 neighborhood of the endometrial surface. All the nuclei are 

 basal, and no cilia have been recognized. 



The secretion of these glands is a clear, cell-free, lymph-like 

 fluid which is delivered into the uterine cavity and absorbed by 

 the vesicles. The animal pole, as we have just seen, is at the 

 end of the sixth day the first part of the vesicle to exhaust 

 1he supply of the albumen above it, and must from this time 

 forward depend upon this secretion for nourishment. The 

 rest of the albumen supply disappears very shortly afterward 

 and the entire vesicle is thereafter nourished only by the secre- 

 tion of the uterine glands. 



