adS ASTACOCROTON, A KEW TYPE OP ACARID, 



one another. The nuclei are characterised by the possession of several rounded 

 nucleoli. The basement-membrane has no recognisable structure. It and the 

 basement-membrane of the mesenteron are in intimate apposition, but are not 

 fused, a definite cleft being distinguishable between them in some places, and the 

 dorso-ventral muscles already referred to in connection with the mesenteron inter- 

 vening in others. 



10. Reproductive system. 



'Ihe female genital aperture has, as ah-eady stated, a form which is very usual 

 in the Acarids, viz., that of a longitudinal slit bounded by a pair of vertically- 

 placed chitinous plates. The passage (vagina) into which it leads is surrounded 

 by a thick mass of muscular fibres, and the cavity is almost obliterated in most 

 sectioned specimens by the close apposition of the lateral walls. In front the 

 cavity opens out and then bifurcates, each of the two lateral vaginae thus formed 

 opening into the corresponding division of the uterus. 



In young specimens in which there are no fully formed ova and in which the 

 uterus is empty, the latter is divided, except in front, into right and left 

 cavities by a median vertical partition. In mature specimens with the uterus 

 packed with eggs, this partition only remains complete in the posterior region; 

 further forward it breaks down, only a remnant at most of its dorsal part per- 

 sisting. Further fonvard still it completely disappears and the uterus presents 

 an undivided cavity. 



The wall of the uterus is composed of a single layer of cells supported upon 

 B basement membrane. In the mid-ventral region the latter alone persists. In the 

 lateral regions the cells become vertically elongated. It must be to the activity 

 of these cells that the formation of the thick and complicated egg-shells is due, 

 since there are no other elements that could be concerned in this process. 



The ovary (PL xxxvii., figs. 15 and 16, ov; fig. 17) lies mainly on the ventral 

 side of the uterus with the ventral wall of which, here for the most part com- 

 posed merely of basement membrane, it is intimately united. But in front it .ex- 

 tends round it to the dorsal side and is prolonged a little distance in front of it 

 in the region just behind and between the coxal glands. The ova, developed in 

 the substance of the ovary in the manner subsequently described, then project- 

 ing outwards in fhe stalked stage, later become free in the surrounding cavity 

 (which is simply the body-ca,vity) and there gTow to their full size. Communi- 

 cation between the body-cavity and the interior of the uterus is effected by a pair 

 of apertures situated far forward just behind the coxal glands. The free 

 (outer) stirface of the ovary is covered with a very thin layer (not definitely re- 

 presented in PI. xxxvii., fig. 17) which is prolonged over each of the developing 

 ova — Henking's tunica propria ovarii. 



Although it is impossible to follow the details of the oogenesis in the 

 material at present available, there are a few important points to pe noted. Tlie 

 ova appear in the substance of the ovary — a layer about .05 to .07 mm; in 

 thickness. The cells, as they near the outer surface, become separated into two 

 sets — (1) those destined to become ova and (2) those destined to become stalk- 

 cells. The former become larger and come to project on the free surface. The 

 latter remain small with denser protoplasm and very small nuclei, and form 

 groups of about five or six beneath the growing ova. Each group is developed 

 into a stalk with about a dozen nuclei, embedded at its base in the substance of 

 the ovary, but quite sharply cut off from the latter, and bearing a young ovum 

 at its free end. Such a stalk probably plays the part of a nutrient organ, re- 



