EMBRYOLOGY. 87 



a more skilled observer would have made them 

 out. 



" Indeed, it is a most difficult thing to discover their 

 rudiments in the adult; I failed, at the time these 

 observations were made, to detect them, though since 

 then I have succeeded in making out their structure 

 and relation to the surrounding parts of the mouth. 



" The cephalic lobes are very distinct, and posteriorly- 

 defined by a slightly marked suture from the postoral 

 portion of the head. They are deeply cleft by the 

 median line of the body. There are no indications of 

 the basal tergites of the head, the segments to which 

 they belong being not yet differentiated from the 

 thorax. 



" A later period (figs. 13, 13 a) is characterized by 

 the difierentiation of the head as a distinct region of 

 the body, the posterior portion or postoral division of 

 the head (including the mandibular and first maxillary 

 arthromeres) uniting with the cephalic lobes to form 

 the head, which is distinctly seen to move freely on the 

 thorax. The ocelli are eight in number and arranged 

 obliquely in pairs, being situated on an elongated, oval 

 area just above the base of the antennae. The two 

 pairs of rudimentary postoral appendages, comprising 

 the mandibles and maxillge, are now greatly increased in 

 size, both of much the same size and form, except that 

 the hinder pair are divided by a slightly' marked articu- 

 lation which is not observable in the mandibles. The 

 basal division of the maxilla probably represents the 

 cardo and stipes together, the distal articulation 

 representing the future palpus, galea, and lacinia. 

 When seen in a front view we can better observe the 

 relations of these organs to the labrum. This latter 

 partially overlaps the mandibles on their inner edge, 

 while the maxillge are more external, though partially 

 covered in front by the mandibles. 



" The front of the head is so entirely different from 

 what it is in the adult, that certain points demand our 

 attention. It is evident that at this period the develop- 



