A reference handbook of the medical sciences, embracing the entire range of scientific and practical medicine and allied science . the other,the tropho-ectoderm, is homologous with tro-phoblast of the higher mammals and gi\es rise to theexternal covering of the chorion. The establisli-ment of the embryonal area is preceded by theappearance of an inner layer of eeUs in the formativehemisphere. This inner layer of primitive endo-derm is produced according to Hill by cells which aredifferentiated in the outer lajer and migrate of Hills results is afforded by later ob- REFERENC
A reference handbook of the medical sciences, embracing the entire range of scientific and practical medicine and allied science . the other,the tropho-ectoderm, is homologous with tro-phoblast of the higher mammals and gi\es rise to theexternal covering of the chorion. The establisli-ment of the embryonal area is preceded by theappearance of an inner layer of eeUs in the formativehemisphere. This inner layer of primitive endo-derm is produced according to Hill by cells which aredifferentiated in the outer lajer and migrate of Hills results is afforded by later ob- REFERENCE HANDBOOK OF THE MEDICAL SCIENCES Blastoderm siTvations made by Minot (1911) on the blastocyst ofthe opossum. In the Eutheria, the ordinary mammals with whichwe are familiar, including man, the ova are verjsmall ( to niillimcter diameter), are very noorin yolk, and are never covered by any shell or sheU-meinbrane. Rarely, as in tlie rabbit, the egg receivesa coating of albumen during its passage throughthe Fallopian tube. The cleavage is holoblastic, butthe result is very unlike the blastula of the J;y Lttl Fig. 663.— Through Three Stages of theBlastocyst of the Mole, x and z, Zona pellueida; Ec, trophoblast;hy, endoderm; , inner cell mass. (After Heape.) In the marsupials, as stated above, the cleavage isradial and produces an open ring of blastomeres thatgive rise directly to a single layered blastocyst. Inthe Eutheria, on the other hand, the origin of theblastocj-st is indirect. The first two cleavages pro-duce a characteristic four-cell stage in which the blasto-meres have a peculiar cross-sliaped the close of tlie cleavage, the minute embrjo is asolid mass of cells, or morula. In some cases theinner cells of the morula are found to stain differentlyfrom the outer ones, indicating a difference in con-stitution, and it has been suggested that the latterhave grown around the former by a process of over-grow
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