300 CLEAVAGE (SEGMENTATION) AND BLASTULATION 



each other through their irregularity in division but also by their tendency to 

 shift their position with respect to one another. One function of the zona 

 pellucida during the early cleavage period appears to be to hold "the blasto- 

 meres together" (Heuser and Streeter, '29). From the 16-cell stage on, the 

 trophoblast or auxiliary cells begin to form the blastocoelic space, first by a 

 flattening process and later by the formation of a cleft among the cells (fig. 

 145D). The growing presence of the blastocoel consigns the formative or 

 inner cell-mass cells to one pole of the blastula (fig. 145J-L). A blastocoelic 

 space thus is formed which is surrounded largely by trophoblast or nutritive 

 cells (fig. 145K, L). The blastular stage of development of the mammalian 

 embryo is called the blastocyst. 



2) Early Development of the Rabbit Egg. The following brief description 

 pertains to the early development of the rabbit egg up to the early blastocyst 

 condition. 



a) Two-cell Stage. The two-cell stage is reached about 22 to 24 hours 

 after mating or 10 to 12 hours after fertilization. One cell has a tendency to 

 be slightly larger than the other (fig. 144B). (Cf. also figs. 145A, F; 146A; 

 147B, J.) 



b) Four-cell Stage. This stage is present about 24 to 32 hours after 

 mating or 13 to 18 hours after fertilization. The larger cell divides first, giving 

 origin to three cells; the smaller cell then divides. (Cf. figs. 145B, C; 146B, C; 

 147K, L.) The mitotic spindles tend to assume positions at right angles to 

 each other during these cleavages. 



c) Eight-cell Stage. Eight cells are found 32 to 41 hours after mating. 

 One member of the larger blastomeres of the four-cell stage divides, forming 

 a five-cell condition, followed by the division of the second larger cell, pro- 

 ducing six cells. (Cf. figs. 145C; 147M.) After a short period, one of the 

 smaller cells segments, and thus, a total of seven blastomeres is formed. The 

 last cleavage is followed by the division of the other smaller cell, producing 

 eight blastomeres (fig< 144C; compare with fig. 147N). The mitotic spindles 

 of each of these cleavages form at right angles to one another, thus demon- 

 strating an independence and asynchrony. The latter conditions are demon- 

 strated further by the fact that the blastomeres shift their position continually 

 in relation to each other during these divisions. 



d) SiXTEEN-CELL Stage. The mitotic divisions increase in rate, and at 

 about 45 to 47 hours after mating the 16-cell stage is reached (fig. 144D). 

 The cells at the future trophoblast pole begin to flatten, and gradually certain 

 blastomeres are enclosed within. In the macaque monkey, 16 cells are present 

 at about 96 hours after fertilization. 



e) Morula Stage. At about 65 to 70 hours after mating a solid mass of 

 cells is present. This condition is known as the morula (mulberry-like) stage 

 (fig. 144E, F). The trophoblast portion of the cell mass is more active in 

 cell division. 



