EARLY DEVELOPMENT OF JULUS TBBRBSTRIS. 229 



The changes which take place on the tenth day result in the 

 embryo assuming its definite shape. These changes consist of 

 the completion of the ectodermal segmention, the formation of 

 the nervous system, and the formation of the ventral flexure. 

 Eight segments, including the head, are marked off from one 

 another by ectodermal furrows, the last segment being a long 

 one, from which the anal segment will eventually be divided 

 off. Each of these eight mesodermal somites has now acquired 

 a cavity. This is shown in fig. 28, which is a vertical longitu- 

 dinal section through the second segment on the tenth day. 



The two layers are distinguishable, the somatic being chiefly 

 concerned in the formation of the muscles of the limbs. 



The ventral flexure now begins to be formed between the 

 seventh and eighth segments. Its first appearance, shown in 

 figs. 29, 30, is seen quite clearly from the outside through the 

 chorion. MetschnikoS' has described it as occurring on the 

 tenth day in Strongylosoma, which hatched on the seventeenth 

 day, in a more advanced stage than Julus terrestris is at 

 the time of hatching. 



The ventral fiexure is first formed by a deepening of the 

 transverse furrow which forms the division 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. 

 The furrow does not deepen in a direction at right angles to 

 the long axis of the embryo, but in a slanting direction, as 

 shown in fig. 14. The effect of this is that the end segment is 

 bent round against the head segment. The eighth segment 

 just referred to is considerably longer than any of the others 

 except the head, and the tissues show a considerable differ- 

 ence to the tissues in other parts of the body. Even on 

 the eleventh and twelfth days, 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- 

 cord not showing any ganglia but lying on the ectoderm 

 as a thin cord not quite separated from it. At a later period 

 of development the anal segment is constricted off from this 



