692 PROCEEDINGS OF THE ACADEMY OF [Oct., 



product of a division which lies toward the animal pole receives the 

 exponent 1, that toward the vegetal pole the exponent 2. Thus la^ 

 lies nearer the animal pole than la-. If the cleavage is meridional 

 the right cell, as seen by an imaginary observer located at the animal 

 pole, receives the larger exponent. Thus la^-^ lies to the right of la^--. 



The macromercs receive a coefficient corresponding to the number 

 of the quartette to which they last contributed. Thus 4A last gave 

 rise to 4a. 



When cells arise whose origin and fate are similar to those of annelids 

 or mollusks which have already received special names, as, for example, 

 the "trochoblasts" or the "intermediate girdle cells," I have made 

 use of these names. I do not, however, wish to imply that the cells 

 arc necessarily homologous with those to which these names were first 

 applied. 



The animal pole is that point at which the polar bodies are given 

 off, the point opposite is the vegetal pole. 



IV. — History of the Cleavage. 

 (1) Unsegmented Ovum. 



J\Iy observations on the unsegmented ovum have unfortunately been 

 confined entirely to fixed and stained material. Since in each lot of 

 material all stages were found, from the unsegmented ovimi to the 

 larva about to hatch, a considerable amount of time and labor would 

 have been involved and many ova lost in selecting for study the few 

 which were still in an unsegmented condition. The ova are approxi- 

 mately spherical and not elongated in one dimension, as Korschelt 

 describes them in D. apatris. J\Ieasurements of the diameters of six 

 unsegmented eggs were respectively 108 micra, 100 micra, 90 micra, 

 96 micra, 92 micra and 100 micra, giving as the average diameter of 

 the egg 97.66 micra. These measurements nearly approach those 

 given by Korschelt for D. apatris, i.e., Ill micra x 92 micra. 



Closely smTOunding the ovum is a delicate wrinkled vitelline mem- 

 brane. The protoplasm in the living ovum is nearly opacjue, this 

 opacity being due to the presence of minute deutoplasmic spheres uni- 

 formly distributed throughout the cytoplasm. These deutoplasmic 

 spheres give to the stained and mounted ova a darkly granular appear- 

 ance, which in many cases makes both mitotic figures and cell outlines 

 difficult to distinguish. Fig. 1^ shows the ovum just after the extrusion 

 of the second polar body. The latter is spherical in shape and about 

 half as large as the first polar body, which is somewhat ovoid. In 

 the first polar body a faint nucleus can be made out, but none in the 



^See Plates XLIII-XLVIII. 



