DEVELOPMENT OF THE ALBINO RAT 53 



The cells forming the roof of the segmentation cavity do not 

 appear so distinctly flattened as is the case in certain of the 

 vesicles figured in figures 20 and 21. It would appear, there- 

 fore, that at least two factors are operative in the increase of 

 size of the segmentation cavity after its anlage a flattening out 

 and consequent increase of the exposed surfaces of the enclosing 

 cells, and secondly, a cell proliferation; and it would appear that 

 both of these factors may be operative from the time of the 

 beginning of segmentation cavity formation. 



Early stages in the blastodermic vesicle formation in the 

 albino rat have been previously described by Robinson, Christi- 

 ani, Duval, and Widakowich; Selenka's youngest stage is slightly 

 older than any discussed by me. My own observations are 

 wholly in accord with those of Widakowich in so far as his account 

 covers early stages of blastodermic vesicle formation. He dis- 

 cusses and figures, however, only two vesicles, obtained four 

 days after fertilization 'Befruchtung/ in each of which the 

 segmentation cavity presents a smooth and regular outline and 

 is of appreciable size. The observations of the other observers 

 who have considered these stages will be discussed in connection 

 with a very brief presentation of much more comprehensive 

 observations on the mouse in similar stages of development. Of 

 these latter, those of Sobotta ('03) are based on abundant and 

 apparently well fixed material. Sobotta begins his discussion 

 with the consideration of three ova taken from the same mouse, 

 the second half of the fourth day after fertilization, each of which 

 shows beginning of segmentation cavity formation, one of which 

 was cut in longitudinal axis and is figured in his figure 1. This 

 ovum is interpreted as showing that the segmentation cavity 

 arises not as a single space, but as a number of disconnected 

 spaces, which later become confluent and form a single space. 

 A similar observation was made by Van Beneden on the bat, 

 a fact which Sobotta uses to support his contention that the 

 mouse ova studied by him were of normal structure. Melis- 

 sinos gives a number of figures showing early stages in the forma- 

 tion of the segmentation cavity in the mouse. His figures 21 

 and 22 (66 hours) are not unlike' my own figures shown in B of 



