112 bulletin: museum of comparative zoology. 



transverse sections of the same stage, of which 81 is the most posterior. 

 In the blastoderm at this stage there is nothing wortl)y of note except 

 the indentation which marks the former position of the blastopore. The 

 cells in this region are rarely as closely arranged as in the other parts of 

 the blastoderm. 



The mesoblast cells are crowded together, and it is impossible to dis- 

 tinguish in all cases between those derived from the primary mesoblast 

 and those from the " secondary mesoblast." As used in the description 

 of later stages, the term mesoblast includes both the primary and 

 " secondary mesoblast." 



The possibility of origin of mesoblast cells from the blastoderm after 

 the sixth cleavage has been kept in mind during the observations, but 

 there is no evidence of such an origin. The cleavage spindles in all parts 

 of the embryo have been seen, but not one perpendicular to the surface 

 has been detected. Moreover, the mesoblast cells have been repeatedly 

 counted in sections and their nuclei have also been counted in transpa- 

 rent preparations of the entire egg, and there have never been seen more 

 cells than could be accounted for by the division of the eight mesoblast 

 cells described in the sixty-two-cell stage. 



It should be mentioned that by rapid decolorization of specimens 

 stained in borax carmine it has often been found possible to draw the 

 color from the nuclei of the blastoderm cells and stop the reaction while 

 the mesoblast nuclei were still brilliantly stained. With such prepara- 

 tions it is easy to count the nuclei of the mesoblast cells in the entire 

 egg. This method has been employed in all the stages with mesoblast. 



The entoblast nuclei are stained brightly by this carmine method, and 

 are easily identified in transparent preparations of entire eggs, as well as 

 in sections. In all stages between that of thirty-two cells and that with 

 about one hundred and twenty cells there is no evidence of division of 

 these nuclei. In these stages only two " resting " nuclei are to be found 

 in the yolk, as shown in Figures 78-80 and 81-86 (Plate 9). Usually 

 in the 120-cell stage the two nuclei are enlarged, while the chromosomes 

 are distinct. Evidently the nuclei are preparing for division, but the 

 spindles are rarely seen until after the blastoderm cells have divided 

 again. In the resulting stage, with about two hundred and fifty cells, 

 four entoblast nuclei are often seen. It does not seem possible that 

 there can have been an overlooked division of these nuclei. Moreover, 

 the origin of the mesoblast cells has been determined to be independent 

 of the two entoblast cells, which are seen in this and in the preceding 

 stage. 



