BULLETIN OF THE UNITED STATES FISH COMMISSION. 139 



between the periods when a given set of cells divide there is a longer 

 interval of repose or rest, which may even in some cases be accom- 

 panied bj' a slight subsidence or depression of the cells which have just 

 divided. Brooks speaks of it in the mollusks as ar contraction of the 

 resulting cells. It seems to me, from what I have seen in the segment- 

 ing eggs of Mya and AJosa, that the term "subsidence" is more directly 

 applicable, since in the act of segmentation the protoplasm appears to 

 have a tendency to become rounded or heaped up in the two new cells, 

 and that afterwards or during the period of rest the protoplasm has a 

 tendency to subside or spread out, as a result of which the segmenta- 

 tion furrows become much shallower. Brooks* has noticed these phe- 

 nomena in ova supposed to be those of the toad fish {Batrachus imi), 

 and alludes to observations in other forms by E. B. Wilson and S. F. 

 Clarke. What first attracted my attention to the matter was the fact 

 that, while ova in the early stages of development were under observa- 

 tion, I was frequently surprised to find, after having left the microscope 

 for a few minutes, that a sudden and rapid change had taken place, 

 while no change whatever was observed previously for the space of an 

 hour or more. Brooks has kept a time record, which I neglected to do, 

 but I can say, however, that in the shad egg, in -which I have mostly 

 observed this phenomenon, the intervals of rest are of much shorter du- 

 ration than recorded by him, showing that he dealt with an egg which 

 developed more slowly. In the mackerel ovum, in which I have had 

 but little chance to observe these phenomena, the intervals of rest are 

 of less duration still, but inasmuch as it develops with three times the 

 rapidity of the shad egg, it is plain that it would be an admirable sub- 

 ject for investigation in respect to this point, and in which micropho- 

 tograi^hy would be an invaluable aid. 



The early stages of segmentation of the mackerel, studied by the 

 writer, unless observed with the microscope, on an unsteady boat, 

 where such observation is almost, if not altogether impossible, were 

 usually too far advanced to keep track of the sets of dividing cells, 

 which were already too numerous, so that they would confuse the 

 student in his attempt to follow the changes marked by intervals of 

 rest between periods of acti\dty. The writer, however, must admit 

 that he has never been able to distinguish the nuclei of the cells as 

 clearly without reagents as figured by Brooks, although working with 

 the most approved apparatus for obliquity of illumination, and with 

 lenses of fine definition; but this may be due to the fact that diiferent 

 species were studied by us. 



STRUCTURE OF THE TESTES AND GENESIS OF THE SPERMATOZOA. 



The testes of the Spanish mackerel are paired organs, like the roe or 

 ovaries of the female, and have much the same position in the abdom- 



* Altematious of Periods of Eest with Periods of Activity in the seo^menting eggs of 

 Vertebrates. Studies Biolog. Laboratory, Johns Hopkins University, Vol. II, 117- 

 118, 1 pi., June, 1881. 



