[116] 



OVUM. 



ing a single nucleus, in others multiple ma- 

 culae. 



The formation of the ova may be observed 

 with ease in any of the smaller isopodous 

 Crustacea. According to Leuckartin Oniscus 

 or Armadillo, it is essentially the same as in 

 the Arachnida. The ova consist at first of 

 germinal vesicles, originating below the epi- 

 thelial lining membrane of the ovarian sac. 

 The yolk-substance first appears as a clear 

 deposit round each germinal vesicle ; minute 

 opaque granules are then formed in this sub- 

 stance, and subsequently the larger albuminous 

 and oil globules gradually make their appear- 

 ance. The vitelline membrane, which is very 

 delicate and structureless, is added at a com- 

 paratively late period, in the Oniscus for ex- 

 ample, when the ova are about y^/' in 

 diameter. 



In many of the Crustacea the ova also 

 acquire a chorion or shell membrane of con- 

 siderable strength. On arriving at the lower 

 part of the female passages, the ova of many 

 genera also receive an addition of a peculiar 

 so-called albuminous secretion, which becomes 

 coagulated in water, and thus, when the eggs 

 are laid, serves to glue them together in heaps 

 or to cause them to adhere to the hinder feet, 

 caudal plates, &c., of the parent, where, as is 

 well known, they remain during the whole 



Fig. 81*. 



Ephippial ovum of the Daphnidce. ( From Haird.} 



The figure represents a profile view of the female 

 of Moina rectirostris (one of the Daphnidas) show- 

 ing at a. the ephippial ovum in its usual place on 

 the back of the animal. 



progress of embryonic development. In the 

 Monoculi and some other Entomostraca, there 

 are marsupia or pouches appended to the 

 genital orifices of the parent, in which the 

 ova are retained during the formation of 

 the young. 



In all the bisexual Crustacea the ova are 

 fecundated while still within the body of the 

 female parent ; but the phenomena and period 

 of this process have not yet been acurately 

 determined, partly perhaps in consequence of 

 the peculiar form and motionless condition of 

 the spermatic corpuscles belonging to the 

 greater number of this class. No micropyle 

 has yet been observed in the crustacean ovum. 



From the observations of several naturalists 

 it is now well ascertained that in the ento- 

 mostracous Crustacea, there commonly occurs 

 a production of young individuals without 

 impregnation, somewhat in the same manner 

 as previously described in the Aphides. " In 

 the Daphnia," says Dr. Baird*, "it is now 

 clearly ascertained that a single copulation is 

 sufficient, not only to fecundate the mother 

 for life, but all her female descendants for 

 several successive generations ;" and it was 

 considered probable by Jurine, that in some 

 species this might extend to the fifteenth gene- 

 ration. 



In the Daphnia and other similar Ento- 

 mostraca, the ova are transferred from the 

 ovary into a cavity situated below the shell 

 on the back of the animal, which has been 

 called uterus, perhaps erroneously, and there 

 undergo development. But at certain seasons 

 many of these Entomostraca produce ova of 

 a different kind from those now referred to. 

 To these the name of winter or hybernating 

 ova has been given, as they appear to be 

 adapted, from the strength and impermeability 

 of their external coverings, to resist the in- 

 jurious effects of cold and other atmospheric 

 influences during the winter season. These 

 ova are generally in smaller number than those 

 of the ordinary kind, frequently two, some- 

 times only one; and they are contained and 

 undergo development in a peculiar case, which 

 is formed on the back of the animal below 

 the shell, nearly in the same situation as the 

 matrix for the ordinary ova. This case, which 

 afterwards separates "from or is abandoned 

 by the animal, forms a sort of hump or saddle 

 on its back, and has hence been named the 

 ephippium, and the eggs have been called 

 ephippial ova. These ephippial ova, according 

 to Baird, are already fecundated by the original 

 impregnation of the female parent, and do not 

 require, for themselves nor for their progeny 

 for several generations, any renewed or special 

 impregnation. 



It appears from the observations of Jurine, 

 Strauss, and Baird, that at the time when the 

 ephippial ova are about to be formed, a sudden 

 change takes place in the appearance of the 

 ova, by the deposit of a quantity of dark granu- 

 lar substance. This appears to be transferred 



* Nat. Hist, of the British Entomostraca, Ray 

 Soc. Public., p. 79. 



