354 FREDA M. BACHMANN. 



differentiated early in development while others arise from epi- 

 thelial cells after the genital fold is formed. 



Eigenmann ('92) in studying Micrometrus aggregatus found that 

 the sex cells first become conspicuous in the mesoblast at a stage 

 before any proto vertebrae are formed. They can be seen in 

 earlier stages but do not stand out so prominently from the other 

 cells. Judging from their size they are probably segmentation 

 cells of the fifth generation. The sex cells can first be distin- 

 guished about the time the blastopore closes. The earliest ones 

 are before the mesoderm is split off from the entoderm. They 

 differ from the surrounding cells in having well-defined rounded 

 outlines and in the distribution of the chromatin in the nucleus. 

 In the sex cells the chromatin is uniformly distributed in small 

 granules; in the surrounding cells it is collected in two or three 

 masses. Not all the eggs of this stage show the sex cells equally 

 well. In some eggs in which the blastopore is closing none can be 

 made out with certainty. In some eggs many large cells were seen 

 in the ectoderm but the inner cells of the blastoderm segment more 

 rapidly toward the close of segmentation than the outer cells 

 and these outer ones become distended through intracellular 

 digestion of the surrounding ovarian fluid. But it is still possible 

 that the sex cells arise in the ectoderm. If they are segregated 

 as early as the fifth generation, i. e., before there is any dif- 

 ferentiation into ectoderm and entoderm, it seems to be of no 

 great importance whether at the time of the separation of the 

 blastoderm into ectoderm and entoderm the cells lie in its inner 

 or outer portion. There is no change in number or size up to 

 the time the larvae are 2.5 mm. long. The length over which the 

 majority of cells is distributed is about 0.20 mm. 



Eigenmann ('96) continued his observations on the sex cells in 

 Micrometrus aggregatus. He finds that the striking feature is 

 the asymmetry of the two sides and the variation in number and 

 position of the cells in different larvae. He finds the nuclei 

 in the sex cells of embryos 5 mm. in length have become some- 

 what larger and the nucleoles also so that in the period of apparent 

 inactivity there have been histogenic changes just as truly as 

 the changes in the somatic cells although there is no division. 

 In later stages the tissues containing the sex cells form a median 



