MYRIAPODA. 



353 



The structure of the duct and of its numerous 

 ovisacs is best seen in those specimens that 

 have not yet arrived at maturity, or in those 

 which have just deposited one laying of eggs. 

 In these individuals the oviduct, to within a 

 short distance of its division into two outlets, 

 is studded with minute ovisacs, each rilled with 

 the rudiments of its minute ovum. Its ge- 

 neral appearance in a female, that has recently 

 deposited its eggs, is completely botruoidal, 

 very like the ovary of Birds, some ova being 

 always fully developed, and ready to pass into 

 the oviduct, while others are in various stages 

 of developement, many of which are imper- 

 ceptible except with the aid of a powerful 

 lens. 



But the most remarkable condition of the 

 female organs in the Julidse is their double 

 vaginal outlet, as in Crustacea, although the 

 oviduct itself is a single tube until near its 

 termination, where it is divided into two short 

 canals, winch from a slight opacity at their 

 base, where they join the single duct, appear, 

 when seen by transmitted light, to be separated 

 from it by a valve or duplicature of the lining 

 mucous membrane. The vaginal orifices are 

 simply two nipple-shaped portions of the tegu- 

 ment, with somewhat oval apertures sur- 

 rounded by a corneous ring, from which is 

 developed a circle of minute hairs. They are 

 situated on the under surface of the fourth 

 segment of the body, and correspond in posi- 

 tion to the insertion of the legs in the third 

 segment. 



Ova. We have already seen from Mr. New- 

 port's description of the female generative 

 system of Julus, that the ova are formed in 

 separate ovisacs, from which they issue com- 

 pletely constituted eggs, into the cavity of the 

 ovarium or common duct, through which they 

 are expelled from the body after impregnation ; 

 and we now proceed to lay before the reader 

 the important results of the investigations of that 

 distinguished anatomist relative to the struc- 

 ture of the ova themselves, and the progress 

 of embryonic developement. The existence 

 of the ovisacs in Julus as single isolated cap- 

 sules on the exterior of the oviduct, in each 

 of which a single egg is produced, is, Mr. 

 Newport observes, a circumstance particularly 

 favourable to a minute examination of the 

 ovum in all its states, especially as ova are 

 found at the same time in every stage of de- 

 velopement. The smallest ovisacs appeared 

 like very minute glandiform bodies, developed, 

 as it were, immediately from the structure of 

 the duct itself, and in these the rudiments of 

 the future egg had already begun to be pro- 

 duced. The smallest rudiments of eggs ex- 

 amined were of an elongated shape, and as yet 

 not more than three, or at most four blood- 

 globules in diameter. They appeared already 

 to have distinct parietes, and to be filled with 

 very minute graniform cells of a uniform size, 

 slightly opaque, and of a yellow colour. The 

 diameter of these cells, as nearly as could be 

 ascertained by direct comparison, was equal to 

 about one-third of that of a blood-globule. 

 In the midst of these cells there was a larger 

 but much more delicate structure of a circular 



form and equal in size to about two of the cells, 

 but whether this was the germinal vesicle or its 

 macula could not be determined. Other ovi- 

 sacs twice the size of the foregoing were filled 

 with similar contents, and from the opacity 

 and yellow colour of the graniform cells, it 

 was evident that they constituted the yelk in 

 one of its earliest stages. At a later period 

 both the yelk and its including vesicle are in- 

 closed in a distinct membrane the membrana 

 vitclli, and before its escape into the oviduct 

 all the parts of a perfect egg, namely, the yelk, 

 the germinal vesicle with its macula, the mem- 

 brana vitel/i, the albumen, and likewise the 

 shell lined by the membrana externa or chorion, 

 are completely formed. 



Evolution of the embryo. The develope- 

 ment of the young Julus Mr. Newport di- 

 vides into several distinct and well-marked pe- 

 riods, during each of which phenomena are pre- 

 sented of the utmost interest, both to the phy- 

 siologist and in an entomological point of view. 



The first period extends from the deposition 

 of the egg to the gradual bursting of the shell, 

 and exposure of the embryo within it, occu- 

 pying the space of twenty-five entire days, 

 during which the egg acquires a sensible in- 

 crease of bulk. 



On the nineteenth day there was a complete 

 alteration in its form; it was more obtuse at 

 both ends, and had become much larger, and 

 the outline of the embryo, coiled up within the 

 shell and nearly filling the whole interior, was 

 very distinct, although, as yet, there were no 

 rudiments of limbs or even of a division of 

 the body into distinct segments. On the fol- 

 lowing day, the twentieth, the outline of the 

 embryo was more apparent, and on its concave 

 or ventral surface there were faint traces of a di- 

 vision of the body into six segments (fig. 317*). 

 Up to this period Mr. Newport 

 was unable to detect any funis 

 or umbilical cord attached to 

 the embryo, although, in conse- 

 quence of Rathke's observations 

 in Crustacea, such a structure 

 was particularly sought for, the 

 whole body still appearing to be 

 formed of cells of different sizes. 

 From this time the egg be- 

 came every day larger until the twenty-fifth 

 day, when it was greatly distended and' began 

 to assume a kidney-shaped appearance, and 

 commenced bursting longitudinally in the me- 

 dian line of the dorsal surface, the back of 

 the soft and perfectly white embryo gradually 

 pressing through the opening. 



In the second period of developement the 

 embryo is exposed to a" new medium, and 

 perhaps derives the means of its further growth 

 from external sources, although it is still enve- 

 loped in the foetal membranes and retains its 

 connection with the shell. 



The liberation of the embryo is a remarkably 

 slow process as compared with the escape of 

 other animals from the egg. In Mr. New- 

 port's observations, from ten to twelve hours 



* This and the succeeding figures are copied 

 from Mr. Newport's paper, before quoted. The 

 objects have been magnified twenty-five diameters 



Fig. 317. 



