No. I .] POL YCHOERUS CA UDA TUS. i 5 9 



reagent is a mixture of equal parts of absolute alcohol and 

 glacial acetic acid, and even this is very uncertain in its effects, 

 some ova being fixed and losing their color almost immediately, 

 while others are affected very slowly. Examination shows 

 that in these latter the cell structure is much injured, or, more 

 generally, totally destroyed. The reason for this uncertainty 

 is probably due to the difficulty of removing all of the mucus 

 capsule by which they are protected from the salt water. The 

 inner part of this capsule is so transparent that its presence 

 can hardly be detected. 



Ova which have been cut out of the parent are of a delicate 

 yellowish white color, and no trace of pigment is to be seen in 

 them. When laid they are about .06 X .04 mm., and a few 

 flecks of a reddish yellow pigment may be seen scattered 

 over the surface. Examination with an oil immersion shows 

 that these pigment granules are between 2 and 3 /la. in size, 

 and are, roughly speaking, double spheres in shape. Fig. 32 

 represents these granules, a, seen from the side, and b from 

 the end. The form, as well as the curious movements and 

 change of position which they undergo, suggest that they are 

 some form of alga, but I have not been able to demonstrate 

 that such is the case. 



In the newly laid ovum they are few in number, but may be 

 seen, even with a fairly low power, lying on or near the surface 

 of the ovum. After the ovum is laid they begin to multiply 

 and apparently to migrate from within toward the surface 

 of the cell. As will be related in the description of the 

 early segmentation, they form a girdle about the ovum which 

 heralds the first cleavage, and in every successive cleavage 

 up to the ten-cell stage the line through which the cleavage 

 plane will pass is thus marked out by pigment granules before 

 the cell divides. Nusbaum (6) has shown that the pigment 

 granules in the cells in the tail of the tadpole are moved about 

 in a somewhat similar manner, always moving or being moved, 

 within the cell in which a spindle is forming, in such a manner 

 that they form an equatorial plate in the same plane as the 

 chromatosomes. Further, when the cell divides, the surfaces 

 of the two daughter cells lying in contact are covered with 



