4 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 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 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- 



