DEVELOPMENT OF THE OPOSSUM 7 



The first cleavage plane is, of course, meridional and in this 

 first division the cytoplasm of the two blastomeres round up 

 in such a fashion as to cast off bodily the entire polar mass of 

 yolk. The second cleavage plane which is also meridional, 

 passes at right angles to the first, leaving the yolk body partly 

 enclosed by the upper, more pointed apexes of the four blasto- 

 meres. These are, therefore, radially arranged and do not shift 

 their position, as do the blastomeres of Eutheria in the corre- 

 sponding stage. The third cleavages are again meridional, divid- 

 ing the egg into eight equal blastomeres arranged in an open 

 ring near or a little above the equator of the egg. Each cell 

 again exhibits a well-marked polarity, both in greater deuto- 

 plasmic content of the lower pole as compared with the upper, 

 and in the presence of the nucleus somewhat toward the upper 

 pole. 



In view of the subsequent history of the egg the author is fully 

 justified in calling the upper the formative and the lower the 

 non-formative pole of the egg; for the fourth cleavage is hori- 

 zontal and divides the eight blastomeres quantitatively and 

 qualitatively into an upper ring of eight, smaller, lighter-staining 

 cells and a lower ring of larger, more darkly staining cells. All 

 of these facts are fully evidenced by a set of superb photomi- 

 crographs. According to the author's interpretation, the upper 

 ring of cells is destined to form the embryonic portion of the 

 blastocyst; the lower, the non-formative portion. 



For a time the rapidly multiplying cells of each ring occupy 

 a more or less clearly horizontal position somewhat above the 

 equator of the egg. They soon reach the periphery, however, 

 that is, they become applied to the inner surface of the shell, 

 and the formative cells proliferate toward one pole, the non- 

 formative cells toward the opposite pole of the egg. The cells 

 finally meet at the poles, and thus complete the unilaminar 

 blastocyst. By this time the entire albuminous layer has dis- 

 appeared, but considerable yolk still remains in the cavity. 

 During cleavage the egg grows from about 0.35 mm. to about 

 0.45 mm. in diameter and the shell becomes somewhat thickened 

 (text fig. 4 G). 



