BZ W. A. HASWELL. 339 



placing the cells of the nutrient chambers of the ovaries of certain Insects. But 

 before the ova have increased greatly in size, and long before the yolk has begun 

 to be formed, the ovum has developed over its entire surface a definite though 

 thin membrane with the appearance of chitin, which cuts off the stalk and must 

 interfere to some extent with free absorption. The original investment mean- 

 while disappears. The younger stalked ova (PI. xxxvii., fig. 17, 1-4) are quite 

 devoid of yolk, but each contains in its cytoplasm a mass of substance having 

 staining reactions very similar to those of chromatin, 'lliis, corresponding to a 

 "yolk-nueleus," at first surrounds, or partly surrounds, the nucleus; then be- 

 comes aggregated on one side assuming a variety of forms usually analysable 

 into twisted anastomosing threads. Later it becomes broken up into small masses 

 and dispersed through the cytoplasm, and long before the detachment of the ovum 

 from its stalk, yolk-spherules make their appearance. At first they are only 

 developed in the outer zones, leaving a large, shai-ply defined, yolkless central 

 area (PI. xxxvii., fig. 17, 5) ; but later they extend uniformly throughout the pro- 

 toplasm and increase greatly in size from .002 mm., when first clearly distinguish- 

 able, to .015 mm. in the ripe egg. 



How and at what stage fertilization takes place has not been ascertained, 

 nor has any trace been seen of maturation phases. When the ovum enters the 

 uterus it at once becomes enclosed in a thick shell secreted by the elongated 

 epithelial cells of the lateral pai-ts of the uterus. The completed egg is .23 mm. 

 in diameter. The egg shell is about .015 mm. in thickness and consists of three 

 layers, an outer very thin, a middle, the thickest, made up of radially elongated 

 rod-like elements, and an inner which is the original ehitinous investment of the 

 ovum. About fifty of these ripe eggs accumulate in the uterus of a mature 

 female. Their further history has not yet been followed. 



The relations between the ova, the body-cavity and the uterus in Astacocroton 

 seem, if we have regard to the statements in general works such as Warburton's 

 "Arachnids" of the "Cambridge Natural History," or Marie Daiber's "Arach- 

 noidea" of Lang's "Lehrbueh," to be quite without parallel in the Arachnida; but 

 if we look more closely into the descriptions and figures of certain of the original 

 papers we are forced to the conclusion that the relations in question must in 

 certain other groups of the Acarida be essentially the same as in Astacocroton. 



Thus in his account of Bdella Michael (1894-97, p. 516) states "1'he ova 

 are foitned and more or less matured in short pedunculated cysts, each ovum ap- 

 parently forming its own oocyst by pushing out the exterior tunic of the ovary, 

 thus forming a sac in which the ovum lies. Exactly how the ovum gets from the 

 oocyst into the oviduct is not by any means clear to me in Bdella, or, indeed in 

 many of the other Acarina, although it is evident in the Oribatidae and most 

 ■Gamasidae." 



His figures 24 and 25 show clearly enough that before the ova represented 

 can reach the interior of the oviduct they must first become detached from their 

 peduncles and enter by apertures in the wall of the oviduct or uterus. Precisely 

 the same holds good of Henking-'s (1882) Figs. 14-1(3, representing the female 

 reproductive apparatus of Trombidium. 



Under the heading "Glands of unknown function" Michael (1895) in his ac- 

 count of Thyas petrophihts describes as follows a. pair of glands which occur in 

 both sexes: — 



"Lying immediately below the lateral portions of the hollow square of the 

 ventrieulus immediately above the genital organs in both sexes, and about the 

 jniddle longitudinally of tlie latter organs, exist a pair of almost globular or 



