THE METAMORPHOSIS OF FLIES. 611 
talline body, a nervous thread and cortical substance. The pig- 
ment layer begins to form and is finished, and the bulb sends out 
the ganglion cells at the base of the chambers of the eye. The 
nervous centres also take on their last definite form, the hinder 
part of the ventral cord, which already in the first period had ex- 
tended out from the infra-cesophageal ganglion, and had extended 
back into the abdomen, now unites with the thoracic knot. 
similar longitudinal commissure unites it with the infra-cesophageal 
ganglion. At the last moment the central portions send out 
nerves to the sides into the thoracic muscles and into the limbs, 
n which during the tenth and eleventh day the muscles begin to 
form, and afterwards hindwards into the abdomen. 
Of the larval organs only the dorsal vessel is destined to pass 
over into the last division of the developmental period, but it still 
suffers a total transformation. A process of fatty degeneration 
Similar to that which took place in the alimentary canal occurs, 
and on the 12th day it assumes a new form and organization. 
Meanwhile it is not capable of performing its functions, as the 
Want of a histologically perfect system of muscles proves. 
The tracheary system is completed last of all. The first posi- 
tive condition is assumed on the 15th day, and by the 17th it is 
generally entirely formed. The trunks arise for the most part by 
means of the masses of nuclei out of the originally solid series of 
tells, the terminal branches of the organ out of a single cell ; 
e hollow Space between them will form the cavity of the trachea, 
while they branch out by growing outwards. Yet these cells may 
_ “tthe most part be traced back to the masses of nuclei, but soon 
: and especially within the inner of the bundles of primitive muscles 
7 of the thorax, they arise from an organization of the histological 
4 EA elements at hand, ie. the muscular nuclei. This remark- 
eet PEENE E E AEA E T O U re E S E ES, ee 
act does not take place without a reaction in the muscular 
e themselves; their sarcolemma disappears and they de- 
| teriorate into fascicles of trachea wanting the spiral thread. 
bal the organs which have trachex intimately connected with 
have the same developed in the last three days. The tracheæ 
ad Out in the nervous centres, in the bulb of the eyes, and the 
of on canal in its entire course is surrounded by a net work 
a them, They are sent to the rectal papilla in great abundance 
: an with a peculiar development. The dorsal vessel also and the 
¢ 
cavit, muscular system receives tracheæ and likewise the genital 
§ ties with their outlets and accessory apparatus. 
