98 Mr. II. J. Carter on the Sperniatology of anew species of Nais. 



The ovum of this worm (fig. 39) is elliptical and slightly bent 

 upon itself. It averages about l-55th part of an inch in length, 

 and consists of a transparent, coriaceous shell (a), terminated by 

 a thickened, irregular, papillary portion at each end, from the 

 inner aspect of which a kind of chalaza (b) is continued on to 

 the yelk-bag (c). 



Each yelk-bag contains two yelks (d, d), which respectively 

 undergo more or less irregular duplicative subdivision, of which 

 the following is a summary : viz. during the first stage, or that 

 of the larger fissuration, the mass becomes triglobular (39 d), 

 after which only one of the divisions appears to undergo minute 

 division, while the other two either remain passive or undergo 

 what may be termed crypto-division, for their substance certainly 

 passes into minute cells, in whatever way this may be effected 

 (40, 41). As the fragmentation thus goes on, the trilobate mass 

 becomes elongated, by the two unfissurating lobes uniting more 

 intimately to form one part, while the fissurating one forms the 

 other (42) ; and the two extremities of the embryo becoming 

 approximated like the ends of a horse- shoe, it thus lies confined 

 by a delicate membrane, in a somewhat compressed globular 

 form, with a notch in one part of the margin (43). 



The notch now extends inwards towards the abdominal limit, 

 when the two halves of the worm thus become separated, and 

 the part which underwent the visible fissuration appeared to me 

 to be the head. The young Naides now burst through the deli- 

 cate cell which appears to surround them respectively, and, be- 

 coming free in the cavity of the shell (44), travel round it for 

 some time until they have gained sufficient strength to force an 

 opening through one end of it, when they thus make their exit. 



I had not many opportunities of watching the development 

 of the embryo (which occupies about five days), because I only 

 discovered the eggs of the Nais mentioned towards the end of 

 " the rains," when the Glceocapsa was dying off, and these Naides 

 also appeared to get weaker and perish with it; but out of 

 about two dozen eggs, many of which I got within twenty-four 

 hours after they had been deposited (for they were laid in my 

 room), sufficient observations were obtained to enable me to give 

 the above description and accompanying illustrations. 



How the double yelk is produced, when only one ovum is 

 developed at a time, I am unable to state ; but, from one ob- 

 servation, T am inclined to think that the first line of fissuration 

 determines this, viz. from the masses becoming permanently 

 detached at this time, and the rest of the fissuration going on 

 in them separately. 



On one occasion I obtained an egg which appeared to belong 

 to N.fusca, and contained three or four embryos; but un- 



