230 LECTURE XVIII. 



Such vesicles make their appearance in the capillary beginnings of 

 the ovarian tubes, where they are drawn out to microscopic tenuity. 

 From these extremities the ova successively pass into the wider part 

 of the tubes, and in this course increase in size by the multiplication 

 of vitelline cells around the primitive vesicle : at first the ova are 

 separated from each other by an amorphous granular substance of 

 equal size, which is called a placentida, but lower down by mere con- 

 strictions of the egg-tube : here they acquire a distinct vitelline mem- 

 brane, and then, still continuing to increase in bulk, by the progressive 

 addition of that material which is afterwards to be expended in forming 

 the tissues of the future insect, they reach the termination or con- 

 verging point of the ovarian tubes, and enter the shorter and wider 

 oviducal track : here they receive additions to their external surface 

 from the collateral and accessory glandular organs, and admit into 

 their interior the mysterious principle of the male fluid, which would 

 seem to be assimilated into their substance. 



In this state the ova are excluded in most insects ; but there are 

 some species, as the common flesh-fly (Musca vomitoria), in which de- 

 velopment proceeds, prior to the exclusion of the ovum, to an extent 

 equivalent to the acquisition by the embryo of the form and condition 

 of the young of the viviparous entozoa ; the formative processes 

 closely according with those which have already been traced, at p. 77., 

 in the Ascaris acuminata. The sub-divided and hyalinized cells of the 

 yolk have arranged themselves into the form, and have been trans- 

 muted into the tissues, of a worm, — a worm which, coming from the 

 ^%% of an insect, is termed a grub, maggot, or larva. In a few other 

 insects, again, this stage of intra-uterine development is brief and 

 transitory, a merely passive, embryonic stage, which is left for a 

 second one, commencing by the formation of the head, by a slight 

 excess in the growth of the first three segments of the trunk, and the 

 budding forth from these of rudimental limbs ; the head is then fur- 

 nished with eyes, antennae, and trophi, the wings are developed, and in 

 this state, inclosed in the exuvial skin of the larva, and ready to issue 

 from it as the perfect insect, the young of the forest-fly are brought 

 forth. The process is termed pupiparous generation : the more pre- 

 mature production of the flesh-fly is called larviparous generation. 

 By far the largest proportion of the class of insects^ as I have already 

 said, is oviparous ; and the development of the embryo takes place 

 out of the body of the parent. 



There are many striking and beautiful manifestations of instinctive 

 prescience in the modes of oviposition, and in the location and at- 

 tachment of the ova. Many insects not only provide the germ 

 M'ith the nutritive vitelline mass, or the material for the first develop- 



