THE INSECTA 169 



trito-cerebrum. Eventually the intercalary segment is aborted 

 and disappears, but its existence is of great importance, for there 

 can be little doubt that it represents the somite of the second 

 antennae. The limb rudiments of the third, fourth, and fifth 

 embryonic segments give rise to the mandibles, maxillae, and 

 labium, and their ganglia fuse together to form the subcesopha- 

 geal ganglion of the adult. The sixth, seventh, and eighth 

 somites become the three thoracic segments, and their limb 

 rudiments become the three pairs of thoracic legs. In the 

 remaining eleven abdominal segments lying in front of the 

 telson the limb rudiments disappear at an early age, except 

 in the first and last. Those of the first abdominal segment 

 give rise to a peculiar embryonic organ which disappears at 

 a later stage, those of the eleventh abdominal segment give 

 rise to the cerci. The ganglia of the thoracic and first five 

 abdominal segments remain distinct, but those of the last six 

 embryonic segments fuse together to form the last abdominal 

 ganglion of the adult. 



The coelom sacs are larger in Periplaneta and its allies than 

 in other insects. They become divided into dorsal and ventral 

 moieties, the latter extending into the limb rudiments. The 

 dorsal moieties persist for a long time, and eventually the genital 

 organs, the heart, the pericardial septum, the fat-body, and the 

 muscles of the body are formed from their walls. Their 

 cavities are completely broken up, and give place to the ex- 

 tensive system of blood -spaces which surround and run 

 between the viscera of the adult. The ventral moieties of the 

 coelom sacs disappear at an earlier stage : their walls break up 

 and give rise to groups of irregularly shaped cells whose fate is 

 not clearly understood, but it is probable that they enter into 

 the composition of the limb muscles in the thoracic region. 



As the embryo is formed on the ventral side of the egg, the 

 yolk is naturally on the opposite or dorsal side. For a long 

 time the embryo is little more than a plate on the ventral 

 surface, and the head, body segments, rudimentary appendages, 

 etc., are all formed while it is in this condition, the yolk being 

 enclosed only by the outer of the two embryonic membranes 

 or serosa. At a comparatively late period, when the hypoblast 

 is established, all the three layers of the ventral plate grow 

 upwards, and eventually meet and fuse above the yolk, so that 

 the latter comes to lie entirely in the mid-gut or enteron. The 



