2 BIOLOGY OF THE LABORATORY MOUSE 



tions that have been noted in the rate of development of embryos or in the 

 rate of development or form of separate parts. It should be emphasized, 

 however, that the range of variation in these respects is considerable. 



Wherever it is applicable to the mouse we have in general followed the 

 terminology employed by Patten in the "Embryology of the Pig." 



Contentious material is described in footnotes rather than in the text. 

 Some readers will wish to skip these altogether. A complete bibliography is 

 given at the end of the chapter, including a number of articles not referred 

 to anywhere in the text. 



Fertilization. — By fertilization is meant the entrance of a sperm into the 

 egg. Fertilization in the mouse occurs in the upper end of each oviduct 

 where the eggs are found, usually gathered into clumps, after their discharge 

 from the ovaries. The sperm thus have to traverse the length of the uterus 

 and oviduct to reach the eggs, a process accomplished partly through their 

 own motility but for the most part through a churning action of the female 

 duct. Since the beginning of heat in the female commonly occurs about two 

 hours before ovulation, sperm may already be present in the oviduct when 

 ovulation occurs. 



The egg consists of a sphere of living protoplasm, the vitellus, surrounded 

 by a transparent, non-living membrane, the zona pellucida (Fig. lA). The 

 zona pellucida in turn is surrounded by follicular cells which, however, are 

 dispersed soon after fertilization. Within the vitellus is the egg nucleus, not 

 clearly visible in living eggs such as the one shown in Fig. lA, but easily seen 

 in fixed and stained material. 



Mature eggs within the ovary average about 95 jjl in diameter (outside 

 diameter of the zona). Following fertilization the zona pellucida expands 

 until its outer diameter becomes about 1 13 /x (= .0044 inches). This is just 

 within the Hmits of visibiHty for the unaided eye (35). 



Usually only one sperm enters each egg. Almost immediately after 

 entry, which may occur through any part of the egg's surface, the vitellus 

 shrinks slightly in size and the zona pellucida expands, so that a space forms 

 between them (35, 50). This is the peri vitelline space. At this time only 

 the first polar body has been formed. Within the next few hours the second 

 maturation division occurs and the second polar body is budded off from the 

 surface of the vitellus (Fig. iB). 



Not only the sperm head but also the middle piece and sometimes the 

 whole tail enters the vitellus. The sperm head carries in one complete set of 

 chromosomes from the male parent, while the middle piece contributes 

 mitochondria from this parent. These latter are soon distributed through- 



