TYPES OF CLEAVAGE 303 



f) Early Blastocyst. A few hours later or about 70 to 75 hours after 

 mating, a well-defined cleft within the cells of the trophoblast pole becomes 

 evident (fig. 144G). (Cf. fig. 145D, J.) This cleft or cavity enlarges, and 

 the surrounding trophoblast cells lose their rounded shape and become con- 

 siderably flattened. As the blastocoel gradually increases in size, the forma- 

 tive tissue or inner cell mass becomes displaced toward one end of the early 

 blastocyst, as indicated in fig. 144G, H. The blastocoelic space at this time 

 is filled with fluid, and the blastocyst as a whole completely fills the area 

 within the zona pellucida (fig. 144G, H). The pig embryo reaches a similar 

 condition in about 100 hours after fertilization, and that of the guinea pig in 

 140 hours. 



During its passage down the Fallopian tube, the developing mass of cells 

 continues to be encased by the zona pellucida. The general increase in size 

 is slight. In the rabbit and in the opossum, as the cleaving egg passes down 

 the Fallopian tube, an albuminous coating is deposited around the outside 

 of the zona pellucida (figs. I44G, H; 147A). This albuminous layer forms 

 an accessory egg membrane or covering similar to the albuminous layers de- 

 posited around the egg by the oviducal cells in prototherian mammals, birds, 

 and reptiles. At about 80 to 96 hours after mating, the rabbit blastocyst enters 

 the uterus and gradually increases in size. Implantation of the mammalian 

 blastocyst upon the uterine wall will be considered later. (See Chap. 22.) 



3) Types of Mammalian Blastocysts (Blastulae). The early blastocyst of 

 the rabbit described above is representative of the early condition of the 

 developing blastula of the eutherian (placental) mammal. However, in the 

 metatherian or marsupial mammals the early blastocyst does not possess a 

 prominent inner cell mass similar to that found in the eutherian mammals. 

 Comparing the early blastocysts of the higher mammals, we find, in general, 

 that there are three main types as follows: 



( 1 ) In most of the Eutheria or placental mammals the inner cell mass 

 (embryonic knob) is a prominent mass of cells located at one pole of 

 the blastocyst during the earlier stages of blastocyst formation. (See 



Fig. 147. Early development of the opossum egg. (A-H after Hartman, '16; I-N 

 after McCrady, '38.) (A) Unfertilized uterine egg, showing the first polar body; yolk 

 spherules (in black) within the cytoplasm; zona pellucida; albuminous layer; and the 

 outer shell membrane. (B) Two-cell stage. Observe yolk spherules discharged into the 

 cavity of the zona pellucida. (C) Section through three blastomeres of four-cell stage. 

 Observe yolk within and without the blastomeres. (D) Section through 16-cell cleavage 

 stage. Observe yolk within blastomeres and also in cavity of the zona between the 

 blastomeres. (E) Section through early blastocyst showing yolk and cytoplasmic frag- 

 ments and an included nucleated cell within the blastocoel. (F-H) Early and later 

 blastocyst of the opossum, showing the formative tissue at one pole of the blastocyst. 

 (I) Surface view, fertilized egg. (J) Two-blastomere stage. (K) Cell A has divided 

 meridionally into A, and A,. (L) Cell B has divided into B, and B^. (M) A, and A^ 

 have divided as indicated. (N) B, and Bj divide next as indicated. 



