LECTURES ON EMBRYOLOGY. 



57 



And if we test with this first result the proposed 

 modifications in the general classification of Ar- 

 ticulata, we will find that on this ground Worms 

 (Plate IV) will stand lowest, Crustacea (Plate VI) 

 come next, and Insects highest. 



[See Plate VI, Lecture 6 ] 



Let us now examine the changes which take 

 place in the nervous system of the Caterpillar 

 when full grown, (the changes during the growth 

 of the Caterpillar itself have not yet been investi- 

 gated) till it is transformed into a perfect Butter- 

 fly. We have at first, a nervous system, consisting 

 of a series of equally developed and almost equally 

 distinct swellings (Plate XII, fig. A) in the head 

 two large ones ; next, one small oce ; at about an 

 equal distance, a second ; a third, nearly equally 

 distant: a fourth, somewhat more distant; a fifth, 

 tixth, seventh, eighth, ninth, tenth, eleventh, al- 

 most uniformly equally distant; and then a twelfth, 

 which is nearer the eleventh, making, with the 

 head, thirteen. Now, precisely the same number 

 of nervous swellings which we observe, consti- 

 tute the number of rings existing in the Caterpil- 

 lar. 



Uniformly throughout the family of Lepidoptera, 

 that is to say, among Butterflies and Moths, the 

 body consists of thirteen successive rings : and in 

 the lowest condition of these animals in their 

 caterpillar state the nervous system has as many 

 nervous swellings, one for each ring, almost 

 equally distant from each other, and sending off 

 threads to the parts around in each ring. The 

 general structure and position of the nervous sys- 

 tem is as follows : The swellings are throughout 

 united by double threads, which towards the poste- 

 rior part of the body come so near together as to 

 seem a continuous, thick cord; but properly speak- 

 ing, they consist uniformly of double threads. And 

 in the position of these threads, there are some im- 

 portant points. The anterior ones are above the 

 alimentary canal; the others are below; so that 

 the thread which unites the anterior ones with the 

 second,constitute a sort of collar around the alimen- 

 tary tube (PI XIV). Bat all the swellings are united 

 by double threads, even where the threads come 

 near together and seem to be one continuous cord. 

 I insist upon this point, because it shows the uni- 

 formity of structure of the nervous system in all 

 articulated animals, and illustrates it, even in the 

 structure of the nervous system which has recent- 

 ly been discovered in Intestinal Worms. When 

 discovered, it was supposed that Intestinal Worms 

 had a nervous system so different from Articulata 

 as not to belong to that group. The nervous sys- 

 tem in Worms forms a sort of collar, with swellings 

 around the anterior part of the alimentary canal, 

 from which arise a double row of swellings, con- 

 nected by simple threads, extending backwards. 

 This arrangement is indeed not very different from 

 that of the higher Articulata : let only swellings, 

 with their double threads, be disconnected, and 

 we have the arrangement of Worms ; and let the 



two chains of Worms be united in one, and we 

 have the arrangement of Insects. 



As soon as the Caterpillar undergoes the first 

 change towards forming the Pupa towards be- 

 coming immoveable, before it casts its skin for the 

 last time we see (Plate XII, fig. B) that the third 

 and fourth swellings are brought nearer together ; 

 and also the first and second are brought nearer 

 together; the others remaining in the same relative 

 position and in the same proportional distances 

 apart. 



But as soon as, for the last time, the Caterpillar 

 has lost its skin and assumed that peculiar form of 

 Pupa in which it is motionless, then the nervous 

 system in its longitudinal extension assumes this 

 winding form [Plate XII, fig. C.J It brings the 

 swellings nearer together, the first of them being 

 at this time entirely united with those at the head- 

 [PLATE XII N"ET*VF.S OF BFTTFT!FT.TFS ] 



In the following stage, (Fig. D ) the 2d ; 3d, 4th and 

 5th swellings are brought nearer the head,whils 

 the 6th, and 7th, disappear entirely during the pu- 

 pa state, and with them disappear also the lateral 

 threads which arose from them in an earlier con- 

 dition. The second, third, fourth and fifth swellings 

 remain now for some time at the same distance, 

 but are gradually combined in one single and more 

 connected mass. The sixth and seventh, disap- 

 pear. The eighth, ninth, tenth and eleventh re- 

 main at equal distances. And if we compare this 

 condition with the perfect insect, we can see that 

 these few anterior swellings, though arising from 

 five distinct ganglia, will send the nerves to the 

 parts answering to the chest. A region liehind, 

 with the long medial thread without lateral nerves, 

 is the region where the separation between the 

 chest and abdomen will take place. Before the 

 Pupa passes into the state of the perfect insect, 

 the approach of the swellings number two, three, 

 four and five is still increased. So that there are 

 now only three regions of distribution of the ner- 

 vous centres : the head with one large mass; next, 

 the chest with separted, though approximated 



