Excerpt from Memoirs of the Wistar Institute of Anatomy and Biology, Vol. 26: The Development of the Albino Rat, Mus Norvegicus Albinus

As concerns the process of segmentation in placental mammals, there are still lacking sufficiently comprehensive observations embracing a number of forms to enable a clear and succinct presentation of the rate of blastomere formation, the cytomorphosis of the cells, and of the relative position of the several segmentation stages in the genital tract. This is no doubt owing to the difficulty of obtaining the necessary material timed so as to admit of proper staging, and the impossibility of making extended observations on living material. Our knowledge of the phenomena of blastoderm vesicle formation, though comprehended in its general phases, is lacking in detail, except for a very limited number of forms. The process of germ layer formation is of such fundamental importance to a clear comprehension of later developmental stages, both in phylogeny and in ontogeny, that a brief account of observed facts in any one form may not be regarded as wholly without value.

Opportunity presented itself, while stationed at The Wistar Institute of Anatomy and Biology, to collect and fix an extended series of embryological stages of the albino rat. This material has proven sufficiently comprehensive to enable a presentation of the several developmental stages of this mammal, beginning with the pronuclear stage and extending to the stage of the anlage of the mesoderm. For this period, which extends to about the tenth day after insemination, only very few of the essential stages are lacking, though for certain of the stages confirmatory preparations would have been desirable. The material at hand, however, seemed sufficiently complete to present a connected account of the stages it is hoped to cover.

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