76 



Early Development of Julus terrestris. 



[Jan. 21, 



cords not snowing any ganglia, but lying on the epiblast and not 

 quite separated from it. At a later period of development the anal 

 segment is constricted off from this segment, while from its anterior 

 part the future segments formed in the course of development are 

 developed. 



Just before the appearance of the ventral flexure the embryo 

 develops a cuticular envelope over the whole surface of the body. 

 This is the so-called amnion of Newport. Just before the formation 

 of the ventral flexure the nervous system is formed. The first traces 

 of this consist in a thickening of the epiblast on each side of the 

 middle line. This is soon followed by the formation of a shallow 

 furrow between the thickened parts ; this longitudinal furrow 

 corresponds with that described by Metschnikoff: in Strongylosoma. 

 The bilobed cerebral ganglia are formed first, and the nerve cords are 

 formed from before backwards, a pair of ganglia being present for 

 each segment except the last. The posterior portion of the nerve 

 cords is completed at a considerably later stage of development. 

 The nerve cords are widely separated, but are connected by a thin 

 median portion. In later embryonic life they are closely approached 

 to one another, and almost form one cord. 



On the eleventh day the embryo has increased considerably in size. 

 The ventral flexure is complete, and the animal lies with the long 

 <end segment folded closely against the rest of the body, the end of 

 the tail being against the stomodasum. The nervous system is now 

 sompletely separated from the epiblast, and the epiblast has 

 assumed the adult form. It now separates a second membrane like 

 that which is formed on the tenth day. 



The splanchnic layer of mesoblast covers the mesenteron, the 

 stomodeeum, and proctodeum. 



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

 cavity, there are present a number of hypoblast cells. These, as have 

 already been mentioned, give rise to the circulatory system and to 

 various muscles. They may, therefore, be now considered as 

 mesoblastic cells which have been directly derived from the hypoblast. 



On the twelfth day the Malpighian tubes are formed as blind 

 outgrowths of the proctodeum, the nervous system is further 

 developed, and the first rudiments of the appendages begin to appear. 

 Late on this day the animal is hatched with only the rudiments of its 

 appendages. I propose to reserve a full description of this stage for a 

 future paper. 



