EARLY DEVELOPMENT OF JULUS TERRESTRIS. 459 
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. Metschnikoff 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 flexure 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 
