LECTURES ON EMBRYOLOGY. 



49 



more points of view, under more manifold aspects, 

 than any other animals. And we have here to in- 

 vestigate external changes, as well as internal mod- 

 ifications of structures ; changes of habits, as well as 

 changes of forms; indeed all the successive trans- 

 formations through which these animals gradually 

 pass from their formation in the egg to their perfect 

 condition. 



The embryology of Insects proper has not been 

 so extensively and so fully studied as the embryol- 

 ogy of other classes. There is generally a great 

 difficulty in examining the eggs of insects, owing 

 to the opaque condition of the yolk-substance, the 

 softness and transparency of the primitive germ, 

 and the thickness of the horny envelope which 

 surrounds the e^rg. You see under what difficult 

 circumstances the observer is placed, to have to 

 break up this hard crust without injuring the soft 

 and delicate germ which isi, besides, exceedingly 

 small, and then to distinguish the various forms of 

 their transparent body, resting upon a dark,opaque 

 centre ; circumstances the most difficult for micro- 

 scopic investigation which can be found. And so 

 we have only a few species whose embryonic 

 growth has been satisfactorily examined. 



Professor Kolliker of Zurich, has made those 

 investigations, and I introduce here, (Plate VII.) 

 the diagrams which he has published of one of 

 those series, in order to show how peculiar the 

 mode of growth of insects is, and how different it 

 is from the changes which other animals undergo 

 within the egg. 



After tracing those changes which take place 

 within the egg, I shall proceed to allude to the 

 changes which the Worm undergoes to form a per- 

 fect Insect. The egg itself consists universally 

 among all insects, of a yolk of opaque substance, 

 enclosed in a hard envelope. When the eggs are 

 laid, there is no germinative vesicle, no germina- 

 tive dot, seen withia. The eggs have really un- 

 dergone extensive changes before they are laid, 

 and when laid, the envelope which surrounds 

 them is already thick and opaque. In order to as- 

 certain whether the egg has primitively the same 

 structure as that of other animals throughout the 

 animal kingdom, it is necessary to trace the for- 

 mation of their substance back to the ovary, and 

 examine the young egg, when the germinative 

 vesicle, with the germinative dot, surrounded by 

 a transparent mass of yolk, enclosed in a mem- 

 brane, will be observed, as in all animals ; afad it is 

 only shortly before the egg is laid that a thicker 

 envelope is formed by the addition of layers of 

 more consistent matter, which are successively de- 

 posited in the oviduct around the yolk membrane, 

 to protect more effectually the eggs, which in so 

 many insects have to pass the winter in that con- 

 dition, before the caterpillar or worm is hatched. 

 However, in the investigation of the formation of 

 the egg and its envelopes, there remains much to 

 be done in the class of Insects. 



It is a peculiarity with the eggs of insects that 



they remain a long time after they are laid, before 

 undergoing their regular transformation ; at least, 

 this is the general impression. That, however, 

 regular transformations begin in the winter, and 

 go on during the cold season in this well-protected 

 cuirass, has recently been ascertained by a gen- 

 tleman of this city, Mr. Waldo I. Burnett, who 

 is at present in vestigating successfully this diffi- 

 cult subject; so that the changes taking place in 

 the eggs of various insects are likely to be soon 

 supplied. 

 [PLATE X. INSECTS WITH THEIR LARV^S AND 



The form of the eggs of Insects is exceedingly 

 variable. There are eirgs, for instance, which are 

 attached to a long stem, (Plate X, fig. B) from 

 which they hang down. That stem, however, be- 

 longs not to the egg proper, but is only a part of 

 its external covering. The layers of protecting 

 substance around the egs, are extended beyond the 

 growth of the egg itself; and through these stems 

 the eggs are attached to leaves of trees, resem- 

 bling little fungi or cryptogamic plants, for which 

 they have been sometimes mistaken. The first 

 thing which takes place in the egg after the ger- 

 minative dot and germinative vesicle are gone 

 after the yolk has become opaque, is the forma- 

 tion of a transparent layer of substance all around 

 the yolk, as seen in Plate VII, fig. A, which repre- 

 sents the young animal, or germ, in its earliest con- 

 dition. As soon as this animal coating has grown 



