DEVELOPMENT OF THE ALBINO RAT 101 



ectoderm. This portion in the section figured, passes a little 

 to one side of the mid-sagittal plane. The two sections preced- 

 ing the one figured enclose the mid-sagittal plane, and in them, 

 the group of cells found between primary embryonic ectoderm 

 and entoderm are in closer relation to the ectodermal layer and 

 at all points distinctly separated from the entoderm. They 

 are regarded as having wandered from the primary embryonic 

 ectoderm to the place they occupy, a fact which is more easily 

 ascertained in cross sections of a similar stage, as will appear 

 from further discussion. From a study of very slightly older 

 stages it can be determined that this region constitutes the 

 primitive streak region of the future embryonic area. It is not 

 my purpose at this time and in this communication to give es- 

 pecial consideration to the much discussed question of the 

 origin of the mesoderm in Mammalia. In the rat, this question 

 is complicated by the question of the anlage of the amniotic 

 fold, which separates the proamniotic cavity into amniotic 

 cavity proper and the ectoplacental cavity, the development 

 of which will be considered in a projected contribution. In 

 anticipation of this second publication, however, the following 

 facts may here receive consideration. Widakowich presents 

 in his figure 4, giving only the antimesometrial end of an egg- 

 cylinder obtained the latter part of the 7th day, the anlage of the 

 mesoderm as observed by him. This figure and my own figure 

 31 present almost identical relations, his figure showing only 

 three mesodermal cells between primary embryonic ectoderm 

 and entoderm. His own words concerning the anlage of the 

 mesoderm in the albino rat, with which I find myself in full 

 accord, except as to the age of the egg-cylinder, read as follows: 



Das erste auftreten des Mesoderms beobachtete ich an Keimen 

 vom Ende des 7 Tages. Die ersten Mesodermzellen liegen im Bereiche 

 der vom mesometralen Ende des starker farbbaren primaren embryo- 

 nalen Ectoderm gebildeten Falte. Es kommt hier eine ganz bestimmte 

 Stelle in Betracht, die dort liegt, wo sich spater das hintere Ende des 

 Primitivstreifens befindet. 



There is, however, wide divergence of the views of authors 

 as concerns the anlage of the mesoderm in the rat and mouse. 



