A SOUTH AMERICAN SPECIES OF PERIPATUS. 223 



soon after that the legs begin to grow out, and the embryo 

 begins to assume the form of the adult. 



Figs. 13, 14 a, and 14 b represent whole embryos at this 

 stage. Figs. 14 a and 14 b are drawn from the same embryo, 

 the latter by reflected light, and made transparent by benzole, 

 the former by direct light in alcohol ; fig. 13 is also drawn with 

 reflected light. 



Figs. 14 A and 14 b represent an acorn-shaped embryo cor- 

 responding to Kennel's fig. 12, lying in the unbroken embryonic 

 vesicle; the band across the embryo and the shaded part of 

 fig. 14 A, from which springs the stalk of the embryo from the 

 thickened area of the vesicle wall, called by Kennel the pla- 

 centa. The stalk, which is represented only in fig. 14 b, is drawn 

 too slenderly ; it should be considerably thicker. 



The other embryo (fig. 13) resembles fig. 14 iu every way, 

 except that it has not been removed altogether from the 

 uterus, of which the split half is still seen attached to the 

 vesicle. 



These embryos both measure "24 mm. across, and the vesicles 

 are respectively "8 mm. and *6 mm. long. 



When the embryo has got to this stage the layers begin to 

 be diflferentiated. The mesoderm and endoderm are formed 

 by a proliferation of cells which takes place at what will after- 

 wards be the hind end of the embryo. 



This process is illustrated in figs. 15 a, 15 b, and 15 c, which all 

 represent sections from different parts of one embryo ; fig. 15 a 

 being at the hinder end and fig. 15 c towards the front end of 

 the embryo. These figures show the embryo proper alone, 

 neither stalk nor vesicle wall have been represented. 



In fig. 15 A the embryo is seen to consist of a double layer 

 of long-oval nuclei (ec), from which are afterwards formed the 

 ectoderm cells, and from which at one point (pr.) there is a pro- 

 liferation of cells (en.) filling up the greater part of the cavity 

 of the embryo ; from these cells the endoderm and mesoderm 

 are subsequently formed ; the endoderm cells have a more 

 granular and more rounded appearance than the ectoderm cells. 



Fig. 15 B shows the appearance of the same embryo some- 



