DEVELOPMENT OF THE ALBINO RAT 17 



living rat ova Long found that the constriction of the second 

 polar body may be completed three-fourths of an hour after 

 its inception, then it must be evident that the pronuclear stage 

 extends through a period which exceeds 10 to 12 hours, since in 

 none of my pronuclear stages obtained 24 hours after insemina- 

 tion was evidence of first segmentation spindle observed. 



In order to determine accurately the relative position of the 

 ova within the oviduct during the pronuclear stage and the 

 stages of segmentation, oviducts containing ova were recon- 

 structed after the Born wax plate method. In form, relations, 

 and general structure, the oviduct of the albino rat is essen- 

 tially the same as that of the mouse as described by Sobotta. The 

 oviduct of the rat measures from fimbriated end to termination 

 in the uterine horn from 2.5 cm. to about 3.0 cm. It presents 

 eight to ten fairly constant major folds, the middle group of 

 which is closely applied to the ovarian capsule. The upper or 

 distal folds pierce the capsule, ending in the fimbriated end 

 found within the capsule, while the lower or proximal folds, 

 proximal with reference to the uterine horn, effect connection 

 with the uterine horn. These relations are essentially the same 

 as those described by Sobotta for the oviduct of the mouse. This 

 observer recognizes four segments in the oviduct of the mouse, 

 characterized by epithelial lining, nature and extent of folding 

 of the mucosa, and thickness of the musculature. The first 

 segment, which falls to the infundibulum, presents a thin muscu- 

 lature and high mucosal folds with epithelial lining consisting 

 of relatively short cylindrical cells with distinct cuticular border 

 and long cilia. As characteristic of this portion of the tube 

 there are further described accessory nuclei compressed between 

 the epithelial cells. Only this portion of the oviduct is ciliated. 

 In the second segment, the lumen is large and the folds of the 

 mucosa prominent. They are covered by a non-ciliated epithe- 

 lium, without distinct cuticular border. The musculature is 

 relatively thin. In the third segment the musculature is well 

 developed with circularly and longitudinally disposed cells. The 

 lumen is narrow and the folds are nearly absent, while the epithe- 

 lium is of a simple columnar variety. The fourth segment, not 

 so well characterized, consists of the loops which make con- 



