340 PHYSIOLOGY. 



delicate ducts, which unite beneath the intestine into a short clavate 

 evacuating duct. The commencement of the nervous system consists 

 of two extremely delicate scarcely perceptible filaments into which the 

 nervous matter by degrees accumulates ; they then approach together, 

 and connect themselves at different spots, thus forming the ganglia, 

 and anteriorly the brain, which in the embryo is still very soft and 

 almost fluid, and therefore very destructible. The muscular layers 

 beneath the skin are also indicated, and particularly the head, with its 

 mandibles, the legs and the anal horn become developed, as the most 

 important external organs. In clothed caterpillars insulated hairs 

 appear also upon the skin. We thus frequently see the matured 

 embryo in its convoluted position through the thin egg shell (PI. I. 

 f. 22). After the termination of these evolutions the young larva 

 strives for freedom and greater independency, it bores through the shell 

 at its most delicate part, namely, at the orbit, and then comes forth 

 from out its prison, and immediately commences its first appointed oc- 

 cupation, feeding voraciously. Producing this object many larvae de- 

 vour their own egg-shell immediately after quitting it. 



212. 



In some few insects the exclusion from the egg takes place in the 

 mother's body, and these therefore bear living young. Such insects 

 are called ovoviviparous. 



One of the most common instances of this kind is presented by the 

 Aphis. In these the female bears through the summer living young 

 ones, and in autumn it lays eggs. According to Bonnet, nevertheless, 

 egg germs are found in the ovaries, as in all other insects ; these deve- 

 lope themselves in the duct, here the young creeps forth, and is thus 

 born living. Bonnet assures us that, upon an anatomical inspection, he 

 discovered egg shells and young ones in the duct. According to other 

 observers, viz , Kyber, upon Aphis Dianthi, eggs are never laid, but 

 young ones constantly born, so long as the individual has not copulated; 

 a copulated and consequently impregnated female lays only eggs ; but 

 Bonnet has nevertheless made it probable that the egg laying (as was 

 remarked above, 204,) is the consequence of the colder autumnal 

 temperature, since the eggs more easily bear the intensity of winter 

 than the young. Kyber's Aphis might therefore have continued 

 producing living young ones in consequence of its being kept in a 

 warmed apartment. De Geer, however, observed Aphis Abietis never 

 to produce living young ones, but always eggs. 



