EARLY MAMMALIAN DEVELOPMENT. 85 



In the white rat, as an example, the entire sperm enters the ovum. After 

 entrance the middle-piece shows increased stainability and the spiral thread, 

 which is probably of mitochondrial origin, becomes evident. The sperm 

 head containing the nucleus enlarges and becomes vacuolated and the 

 centrioles and polar rays appear. As the sperm nucleus forms the male 

 pronucleus, the chromosomes remaining after the second maturation division 

 of the egg are resolved into the female pronucleus. The two pronuclei come 

 in contact near the center of the ovum. 



Observations on mammalian ova are not yet sufficient to justify any 

 conclusions regarding the effect of fertilization on the polarity or internal 

 organization of the cells. It has been therefore impossible to trace in sub- 

 sequent stages of development any relation between egg organization and 

 the planes and constitution of the body in the mammal. 



Cleavage. As in the lower forms, cleavage of the fertilized mammalian 

 ovum is primarily a series of mitotic divisions resulting in a solid cluster of 





a b c d 



FIG. 53. Four stages in the cleavage of the ovum of the white rat. Huber. A, from a model 

 of an ovum in the pronuclear stage, 24 hours after insemination; B, from a model of a 

 2-cell stage, 2 days after insemination; C, from a model of a 4-cell stage, 3 days and i hour 

 after insemination; D, from a model of an 8-cell stage, 3 days and u hours after insemina- 

 tion. 



cells. This has been observed in detail in several mammals: the opossum 

 (Hartman), the white rat (Huber), the white mouse (Sobotta), and a few 

 others. Cleavage in these formsjsjrregulaPand the blastomeres are approxi- 

 mately equal in size (Figs. 53 and 54). The cluster is known as the morula, 

 which corresponds to the similar stage in lower animals. 



In certain mammals, for example the bat (van Beneden), the superficial 

 cells of the morula become differentiated from those in the interior and form 

 a continuous covering layer (Fig. 55, a and b). Following this differentiation, 

 many of the internal cells become vacuolated, the vacuoles coalesce and finally 

 the cells disappear thus leaving a cavity. At one side of this cavity some of 

 the internal cells remain intact and attached in a cluster, known as the 

 inner cell mass, to the covering layer (Fig. 55, c and d). In other mammals, 

 for instance the white rat (Huber), a cavity is formed within the morula by 

 displacement of the central cells toward the periphery, most of the cells 

 moving to one side where they form a cluster which is continuous with the 



