1886.] 



Early Development of Julus terrestris. 



75 



On the seventh and eighth days the keel gradually disappears, and 

 the layers of mesoblast spread round a great part of the embryo, 

 rather more than half way round. On the ventral surface the 

 epiblast cells assume a columnar form, thus giving rise to the ventral 

 plate. 



The mesoblast now becomes thicker on each side of the middle 

 ventral line. Both layers are concerned in this thickening, and at 

 these points the two layers become indistinguishable. Outside the 

 thickenings, that is further away from the middle ventral line, the 

 two layers are closely applied to each other, and to the epiblast as 

 before. The effect of these changes is that the greater part of the 

 mesoblast is now arranged in two parallel longitudinal bands along 

 the ventral surface of the embryo ; these bands being connected across 

 the middle line by a thin portion consisting of a single layer. 



The two longitudinal bands now begin to be constricted off into the 

 mesoblastic somites. The latter are formed from before backwards, 

 and their position corresponds with that of the future segments of the 

 body. The number of the somites is eight, corresponding with that 

 of the eight segments with which the embryo is finally hatched. The 

 somites are at first solid, afterwards a cavity appears in them. 



Early on the ninth day the stomodseum is formed as an invagination 

 of the epiblast near one end of the ventral surface. Shortly after 

 the first formation of the stomodaeum, the proctodseum appears as a 

 shallow somewhat wide invagination of the other end of the ventral 

 surface. 



The body-segments already established by the segmentation of the 

 mesoblast now become more apparent, each being marked by a deep 

 transverse furrow in the epiblast. The hypoblast cells are still 

 present within the yolk, but are gradually becoming collected in the 

 median line, just below the mesoblastic bands. The stomodaeum and 

 proctodeum become more deeply invaginated, extending a considerable 

 distance into the yolk, and at the same time the hypoblast cells 

 begin to form the mesenteron, arranging themselves around a central 

 lumen. 



On the tenth day the ventral flexure is formed by a deepening of 

 the transverse furrow between the seventh and eighth segments. It 

 is, therefore, first formed nearer the anal end of the embryo. As the 

 furrow deepens and the embryo increases in size, the last segment- 

 grows in length. At the same time the embryo curves round towards 

 the ventral surface. The effect of this is that the end segment is 

 bent round against the head. The eighth segment is considerably 

 longer than the others except the head, and the tissues there show a 

 considerable difference. Even as late as the twelfth day, when the 

 nervous system is far developed in all other parts of the body, in the 

 eighth segment the tissues are imperfectly differentiated, the nerve 



