248 G. CARL HUBER 
tilization as observed in placental mammals has in recent years 
been enriched by a number of studies to the extent that for cer- 
tain of the mammals—bat, rabbit, guinea-pig, mouse, and 
rat—the data at hand are sufficiently complete to enable a 
clear and comprehensive presentation, based on observed facts, 
and permit of comparison with similar phenomena as observed 
in other vertebrate and invertebrate forms. 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 diffi- 
culty of obtaining the necessary material timed so as to admit 
of proper staging, and the impossibility of making extended ob- 
servations 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 laver 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 an- 
lage 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 con- 
nected account of the stages it is hoped to cover. The embryology 
of allied forms, especially of the mouse, has received much more 
extended study than has that of the rat, though the develop- 
