4 



BIOLOGY OF THE LABORATORY MOUSE 



side by side, each appearing at this stage as a typical resting nucleus, though 

 the male element is a little the smaller of the two. At the first cleavage 

 division the nuclear walls break down, the chromosomes split longitudinally, 

 and one-half of each split chromosome is carried to each daughter cell. 

 Hence at this division, as at all future somatic divisions, each cell receives a 

 full complement of chromosomes from each parent. 



Cleavage. — Cleavage in the mouse occurs while the eggs are still in the 

 oviduct. The first cleavage occurs about 24 hours after copulation and 



Fig. 2. — Photographs of mouse eggs (X600). .\. Two-cell egg from oviduct 24 

 hours after copulation. Large second polar body and disintegrating first (on opposite 

 side). B. Seven-cell egg from oviduct 48 hours after copulation. Note one cell 

 on left larger than the rest. Division of this cell would give the eight-cell stage. 

 (From Lewis and Wright.) 



results in two cells not quite equal in size (Fig. 2 A). Following divisions 

 occur somewhat more rapidly, giving rise to 4-cell, 8-cell stages, etc., and 

 are usually nearly synchronous in the different cells. Occasionally, however, 

 eggs are found with some divisions completed, others still incomplete, and 

 hence showing an odd number of cells (Fig. 2B). The actual act of division 

 requires only 5 or 10 minutes; the interval between divisions lasts about 12 

 hours. Eggs of 16 cells or more, but in which no cavity has appeared, are 

 called morulae. Eggs usually reach this stage about 60 hours after fertiliza- 

 tion, and pass from the oviduct, through which they have been gradually 

 moving, into the uterus, some 6 to 12 hours later (35). This is subject to 



