224 W, L. SOLATBR. 



what further forward; here all connection between the endo- 

 derm and ectoderm is lostj and the endoderm is growinpf 

 forward in the middle of the embryo at the expense of the pro- 

 liferating cells behind. 



In fig. 15 c, still farther in front, a cavity {mes.) begins to 

 appear in the hitherto solid endoderm j this is the first com- 

 mencement of the enteroh. 



Farther still in front the endoderm thins out somewhat, so 

 as to form a narrow thin band encircling the enteron ; still 

 farther it gradually disappears, so that nothing is left at the 

 extreme head end of the embryo but the ectoderm. From 

 these proliferated cells the mesoderm is also formed later on. 

 Another feature of this stage is the thickening of the ectoderm 

 on one side, or rather, ventrally to form the nerve-cord. This is 

 not marked at the hind end of the embryo, where the thicken- 

 ing extends all round, but is more marked at the front end 

 (fig. 15 c), where on one side the ectoderm is seen to consist 

 of a double layer {n.c), on the other of only one; it is from 

 part of this double layer that the nervous system will be sub- 

 sequently formed. 



The proliferation of cells takes place on the ventral side 

 of the embryo distally to the stalk, which is attached to the 

 dorsal side of the embryo at the head end. This proliferation, 

 therefore, exactly corresponds to the primitive streak of P. 

 capensis as described by Sedgwick; and all that has to be 

 conceded is that in consequence of the extraordinary changes 

 in the early stages due to the small size of the ovum and the 

 absence of yolk, the blastopore of P. capensis described by 

 Sedgwick has disappeared from P. imthurni,^ although its 

 position is still marked by the primitive streak, which replaces 

 the primitive streak + the blastopore of P. capensis. 



After this stage I have not worked at the development of 

 this form ; for one reason I have not had time, for another 

 because the later stages seem to me to resemble those of 

 P. Edwardsii^ and P. capensis as arrived at by Kennel 

 and Sedgwick respectively. 



* See foot-note on p. 214. — Ed. 



