EARLY DEVELOPMENT OP JULUS TEREBSTRIS. 231 



mesenteron. They are divided into ganglia, one pair being 

 present for each segment of the body ; from each ganglion a 

 nerve is given off to the corresponding body segment. The 

 sub- and supra-oesophageal ganglia are almost formed. 



The splanchnic layer of mesoderm covers the mesenteron, 

 tlie stomodaeum, and proctodseum. The median part of the 

 somatic mesoderm lies above the nerve-cord, between it and 

 the gut ; from thence it passes downwards to the borly wall. 

 This arrangement is shown in fig. 34<, which is a transverse 

 section through an embryo of the twelfth day. 



Within the yolk, which is still present in great quantity in 

 the body-cavity, there are present a number of cells remaining 

 over from the hypoderm after the formation of the mesodermic 

 keel, and the mesenteron. These cells eventually give rise to 

 the circulatory system, to the muscles of the segments, in part 

 at any rate, and to other muscles ; they are therefore mesoderm 

 cells. The lumen of the mesenteron is now continuous with 

 that of the stomodaeum and of the proctodseum. 



Fig. 14 shows a longitudinal vertical section through an 

 embryo of this age. 



On the twelfth day the Malpighian tubes grow out of the 

 proctodseum. Their lumen is from the first continuous with 

 that of the proctodseum. They end blindly and are enveloped 

 by the splanchnic mesoderm. 



Fig. 34 is a transverse section through an embryo on the 

 twelfth day. The section is taken through a ganglion in the 

 posterior part of the body. It shows the two ganglia united 

 by a narrow median part and each giving ofi^ a nerve to the 

 ventral part of the body, where the rudiments of a pair of 

 limbs can already be traced. The Malpighian tubes are also 

 sbown. This section also shows the body cavity divided into 

 four compartments by means of thin layers of mesoderm. 

 Late on this day the animal is hatched with only the rudi- 

 ments of its appendages, and I propose to reserve a full 

 description of the stage till a future time. 



