THE EARLY DEVELOPMENT OF THK MAESUPIALIA. 81 



contain deutoplasmic material, which is, however, located 

 mainly in their lower halves. The ensuing fourth cleavages 

 are equatorial, and in correlation with the just-mentioned 

 disposition of the deutoplasm, are unequal and qualitative, 

 each of the eight blastomeres becoming subdivided into an 

 upper smaller and clearer cell, with relatively little deuto- 

 plasm fairly uniformly dispersed through the cytoplasm, and 

 a lower larger, more opaque cell with much deutoplasm, 

 mainly located in a broad zone in the outer portion of the 

 cell-body. A 16-celled stage is thus produced in which the 

 blastomeres are characteristically arranged in two super- 

 imposed rings, each of eight cells, an upper of smaller, clearer 

 cells next the yolk-body, and a lower of larger, denser cells. 

 The former is destined to give origin to the formative or 

 embryonal region of the blastocyst wall, the latter to the 

 non-formative or extra-embryonal region of the same. 



(c) Formation of the Blastocyst. — There is in the 

 Marsupial no morula stage as in Eutheria, the blastomeres 

 proceeding directly to form the wall of the blastocyst. The 

 cells of the two rings of the 16-celled stage divide at first 

 meridionally and then also equatorially, the division planes 

 being" always vertical to the surface. The daughter-blasto- 

 meres so produced, continuing to divide in the same fashion, 

 gradually spread towards opposite poles in contact with 

 the inner surface of the firm sphere formed by the zona and 

 the thickened shell-membrane. Eventually they form a com- 

 plete cellular lining* to the said sphere and it is this which 

 constitutes the wall of the blastocyst. The latter is accord- 

 ingly unilaminar at its first origin, and it remains so in 

 Dasyurus until it has attained, as the result of active growth 

 accompanied by the imbibition of fluid from the uterus, a 

 diameter of 4-5 mm. It consists of two parts or regions, 

 distinct in origin and in destiny, and clearly marked off from 

 each other in later blastocysts by a definite junctional line 

 approximately equatorial in position, viz. an upper, embi'yonal 

 or formative region derived from the upper cell-ring of the 

 16-celled stage, and a lower, extra-embryonal or non- 



VOL. 56, PART 1. NEW SERIES. 6 



