TRACHEATA. 



337 



Formation of the organs and their relation to the germinal 



layers. 



The segments and appendages. One of the earliest phenomena 

 in the development is the appearance of transverse lines indicating 

 segmentation (fig. 186). The transverse lines are apparently caused 

 by shallow superficial grooves, and also in many cases by the 

 division of the mesoblastic bands into separate somites. The most 

 anterior line marks off a prse-oral segment, which soon sends out two 

 lateral wings the procephalic lobes. The remaining segments are 

 at first fairly uniform. Their number does not, however, appear to 

 be very constant. So far as is known they never exceed seventeen, 

 and this number is probably the typical one (figs. 186 and 187). 



In Diptera the number appears to be usually fifteen though it may be 

 only fourteen. In Lepidoptera and in Apis there 

 appear to be sixteen segments. These and other 

 variations affect only the number of the segments 

 which form the abdomen of the adult. 



The appendages arise as paired pouch-like 

 outgrowths of the epiblast and mesoblast ; and 

 their number and the order of their appearance 

 are subject to considerable variation, the mean- 

 ing of which is not yet clear. As a rule they 

 arise subsequently to the segmentation of the 

 parts of the body to which they belong. There 

 is always formed one pair of appendages which 

 spring from the lateral lobes of the procephalic 

 region, or from the boundary line between 

 these and the median ventral part of this 

 region. These appendages are the antennas. 

 They have in the embryo a distinctly ventral 

 position as compared to that which they have 

 in the adult. 



In the median ventral part of the pro- 

 cephalic region there arises the labrum (fig. 

 187, Is}. It is formed by the coalescence of a pair of prominences 

 very similar to true appendages, though it is probable that they have 

 not this value 1 . 



The antennae themselves can hardly be considered to have the 

 same morphological value as the succeeding appendages. They are 

 rather equivalent to paired processes of the pra>oral lobes of the 

 Chsetopoda. 



1 If these structures are equivalent to appendages, they may correspond to one of 

 the pairs of antenna of Crustacea. From a figure by Fritz Miiller of the larva of 

 Calotermes (Jenaische Zeit. Vol. xr. pi. 11, fig. 12) it would appear that they lie in 

 front of the true antemife, and would therefore on the above hypothesis correspond to 

 the first pair of antennae of Crustacea. Biitschli (No. 405) describes in the Bee a pair 

 of prominences immediately in front of the mandibles which eventually unite to form 

 a kind of underlip ; they in some ways resemble true appendages. 



FIG. 186. EMBRYO 

 OF HYDROPHILUS PICEUS 

 VIEWED FROM THE VEN- 

 TRAL SURFACE. (After 

 Kowalevsky. ) 



pc.l. procephalic lobe. 



B. E. 



22 



