50 



PROF. AGASSIZ'S 



sufficiently thick to assume definite outlines, a 

 broad open space is noticed on one side of the 

 germ, through which the yolk is very extensively 

 seen From further changes, it will be ascertained 

 that the continuous mass represents the ventral 

 portion of the animal, and that the free opening is 

 on the dorsal side of the germ. At this earliest 

 stage^ome few changes oi substance have already 

 taken place. The animal layer, when first formed 

 and examined under the microscope, is seen to 

 consist of small cells, which have little dots with- 

 in. At first, there is only one layer of such cells ; 

 then, a second layer is formed, probably derived from 

 the substance of the yolk itself. Then there are 

 three or four such layers, the cells being probably 

 multiplied and increased in number by the burst- 

 ing of the primitive cells, and by the growing into 

 cells of their minute inner dots. 



This seems the more probable, as with the in- 

 crease of layers,the cells becoming more numerous, 

 are also found to be smaller ; so that, when there 

 are four or five such layers, the cells are so minute 

 as to require a higher power of the microscope to 

 examine them ; showing that these cells increase 

 by evolution from the primitive ones. The ap- 

 pearance of a thick animal layer around the yolk, 

 as the first indication of the srerm, with a large 

 open space opposite the main bulk of the embryo, 

 is a peculiar feature of the mode of formation of in- 

 sects, by which they differ widely from other ani- 

 mals. Here, (Plate VII, fig. B) the opening is to- 

 wards one end of the egg, at which end we also no- 

 tice upon one side of the germ, the first indication 

 of a transverse division, marking out the head. 

 Next, (Plate VII, fig. C,) there will be some con- 

 tractions taking place upon the longitudinal axis of 

 the body, dividing the germ into several joints. 

 The first change which takes place in the germ of 

 an articulated animal is, therefore, an indication of 

 the type to which it belongs. It is really an artic- 

 ulated animal before any further indications of a 

 structure are introduced. The first division which 

 takes place goes to indicate the position of the 

 head. At this period, (Plate VII. fig. B), the yolk 

 mass is already reduced to a smaller space. Next 

 the transverse divisions appear, those of the head 

 growing more complicated as represented in Plate 

 VII, fiirs. C, G, H. And then, there is a well defined 

 outline formed below the yolk, (Fig. D) extending 

 to the anterior divisions of the germ, and towards 

 its upper side, going to form the alimentary canal. 

 The mass of the yolk is still more reduced, the 

 membrane which now encloses it from below hav- 

 ing folded itself upwards, so as to assume the shape 

 of a little boat, (Fig. E ) and parcels of yolk re- 

 maining scattered on the sides. At this period we 

 can already observe that the folds on the outside 

 of the body will be transformed into joints. There 

 is a head at the upper end of the germ, and at its 

 lower side there are indications of legs (Fig. E). 

 A wonderful arrangement is now plain, which 

 was first discovered among Articulata by Herold, 



in Spiders, and afterwards confirmed by Rathke in 

 Crawfishes, namely, that in articulated animals the 

 folding of the germ takes place in such a manner 

 as to have the navel upon the back, that is to say, 

 the opening by which the mass of yolk communi- 

 cates with the alimentary cavity has a position 

 strictly opposite to what is observed in other ani- 

 mals. The germ, indeed, folds itself around the 

 yolk, leaving a broad opening on that side of the 

 animal which, in its final structure, will be the 

 back. (PlateVII,Fig. D.) The side opposite the na- 

 vel being the one from whence the feet come out, 

 and that where the opening is observed, being the 

 sideifrom which the wings will be developed. The 

 membrane which was developed below the yolk 

 has now folded itself more extensively upward, and 

 forms an elongated open channel, which finally 

 grows into a closed tube, the alimentary canal, aa 

 it is seen in the animal more fully developed (Fig. 

 F), where there are some parts of the yolk remain- 

 ing in the joints. Before the yolk has entirely 

 disappeared, there is a pair of rudimentary feet 

 developed in the anterior part of the embryo, 

 which will disappear before this embryonic ani- 

 mal has the proper form of the larva to which it 

 gives rise. There are also at the posterior extrem- 

 ity indications of false feet forming, and all along 

 the various joints of the body, which have been 

 successively marked. These are, however, not feet 

 proper, but only stiff hairs. 



From the facts stated above, it is plain that in 

 the class of Insects, after a complete investigation 

 of the growth of the egg of one species, (and indeed 

 of several species) it has been ascertained that the 

 germ is not developed above the yolk, but below, 

 as we have observed it in Kadiata There is not, as 

 in Radiata, a cavity formed below, extending with- 

 in the bodv to the stomach and the mouth ; but we 

 have in this case a germ which is forming below 

 the yolk. Of course, such an egg could be re- 

 versed, and it might be said that there is no differ- 

 ence between the germ of Radiata and Insects that 

 we may just as well turn the egg of Insects so as 

 to have the germ in the same apparent position in 

 both cases. But if we turn in such a manner an 

 egg of an insect, with its germ, we shall find the 

 feet growing out of the upper side, and we shall 

 find the opposite, or lower side, giving rise to a 

 pair of wings. This would only show a re- 

 versed position of the whole; as we may place 

 the fee^ of an Insect upwards, and the wings 

 downwards, and have only an inverted Insect. 

 But by thus changing the external position 

 of the animal, the legs remaining opposite the 

 wings, whether the navel be primitively open 

 between the wings or above the animal, or vice 

 versa, we shall not change the relation of ita 

 parts, in their growth. And so you see, that the 

 articulated animals grow in a position the reverse 

 to that of the Radiata, and undergo successive 

 changes, which at a very early period give rise to 

 those moveable joints which characterize Articu- 



