POLYEMBRYONIC DEVELOPMENT IN TATUSIA 571 



type, show interesting and significant differences. These will be 

 described in the order of their development. 



Blastocyst No. 287. This specimen measures 0.312 mm. in diam- 

 eter, and is therefore larger than several others, but in point of 

 development it is the youngest specimen in the collection. The 

 inner cell-mass appeared both in the living and in the preserved 

 condition as an opaque spot, the inner surface of which showed 

 many elevations, caused by the protruding embryonic cells. In 

 section the mass is seen to be made up of embryonic cells with rel- 

 atively large, distinct nuclei (fig. 6). For the most part each cell 

 is delimited by a cell wall ; but here and there may be found one in 

 which part of the wall has disappeared, or at least the membrane 

 can not be made out with certainty. Another point of interest is 

 the fact that there is a considerable difference in the size of the 

 nuclei. Thus of the eleven nuclei shown in the median section 

 (fig. 6) six are decidedly larger than the remaining five. I am 

 unable to detect any difference in the staining properties of these 

 two types of nuclei, although the cytoplasm of some of the cells 

 which possess the larger nucleus takes a much paler tint than that 

 of the cells with the small nucleus. 



The embryonic knob measures 0.026 mm. deep by 0.055 mm. 

 wide. If we regard the trophoblast of the median section of the 

 blastocyst as the circumference of a circle whose diameter is equal 

 to the diameter of the blastocyst, and using the width of the inner 

 cell-mass as a chord, then we may express the size of the mass in 

 the terms of degrees and minutes, as measured on the circumfer- 

 ence by the subtended arc. In this particular specimen the diam- 

 eter was 0.312 mm., and since the diameter of the inner cell- 

 mass was 0.055 mm., it covered an arc on the circumference equal 

 to 20 18'. The embryonic mass is composed of about 75 cells, 

 as determined by a count of the number of nuclei. They are 

 tightly pressed up against the trophoblast, from which, however, 

 they are sharply cut off by the under surfaces of the trophoblastic 

 cells. 



Blastocyst No. 310 measured 0.360 mm. in diameter, and is there- 

 fore larger than the preceding, and that it is also further developed 

 is evidenced by the fact that there are 136 cells in the embryonic 



JOURNAL OF MORPHOLOGY, VOL. 24, NO. 4 



