80 J. P. HILL. 



polarity, its lower two thirds consisting of formative cyto- 

 plasm, dense and finely granular in appearance, owing to tlie 

 presence of fairly uniformly distributed deutoplasmic material, 

 and containing the two pronuclei, its upper third being 

 relatively clear and transparent, consisting as it does of a 

 delicate reticulum of non-formative cytoplasm, the meshes of 

 which are occupied by a clear deutoplasmic fluid. Study of 

 the process of vitellogenesis in ovarian ova demonstrates that 

 this fluid represents surplus deutoplasmic material which has 

 not been utilised in the upbuilding of the formative region of 

 the ovum. 



The fate of the clear non-formative portion of the ovum is 

 a very remarkable one. Prior to the completion of the first 

 cleavage, it is separated off from the formative remainder of 

 the ovum as a spherical mass or j^olk-body, which takes no 

 direct part in development, though it becomes enclosed in the 

 blastocyst cavity on completion of the blastocyst wall at the 

 upper pole. Its contained deutoplasmic fluid is to be regarded 

 as the product of an abortive attempt at the formation of a 

 solid yolk-mass, such as is found in the Monotreme ovum. 

 By its elimination the potentially yolk-laden telolecithal ovum 

 becomes converted into a secondarily homolecithal, holoblastic 

 one. All the evidence is held to support the conclusion that 

 the jMarsupials are descended from oviparous ancestors with 

 ineroblastic ova. 



(b) Cleavage. — Cleavage begins in the uterus, is total, and 

 at first equal and of the radial type. The first two cleavage 

 planes are meridional and at right angles to each other. 

 The resulting four equal-sized blastomeres lie disposed radially 

 around the polar diameter like those of the Monotreme (not 

 in pairs at right angles to each other as in Eutheria), and 

 enclose a segmentation cavity open above and below, their 

 upper ends partially surrounding the yolk-body. The third 

 cleavage planes are again meridional, each of the four blasto- 

 meres becoming subdivided equally into two. The resulting 

 eight cells form an equatorial ring in contact with the inner 

 surface of the sphere formed by the egg-envelopes. They 



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