DEVELOPMENT OF THE ALBINO RAT 297 
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 
