HABTTS, ETC., OP CERKBRATULUS LACTEUS. 121 



called Ersatz cells, destined to give rise to epitlielium, but 

 still capable of being transformed into neuroblasts. 



After being formed in this ventral position the nerve-cords 

 migrate upward along either side of the body to their final 

 normal situation, carrying with them a thick covering of the 

 neuroblast cells (fig. 22). As soon as the nerve-cords leave 

 the ridges, gland cells begin to appear in the epiblast cover- 

 ing the latter, and it becomes in all respects like that on the 

 outside of the grooves. The grooves then disappear, and 

 leave a smooth epithelial surface. 



The new nerve, therefore, is in no respect an outgrowth 

 from the old one, but results from a new growth of cells 

 which are entirely independent of the old nervous system. 



It hardly seems reasonable that this Nemertean in its 

 embryology should derive its nervous system from mesoblast 

 when in regenerating lost parts it shows such a distinctively 

 epiblastic origin. 



Laeval Development, 

 Sexual Organs. 



The sexes are distinct as in most Nemerteans. There is no 

 perceptible difference in external form or size, but in the 

 breeding season there is a difference in colour, due to the 

 sexual products which show through the body-wall. 



The male then appears cream-coloured, while the female is 

 greyish red, turning to a light chocolate-brown in many 

 cases. 



This difference is clearly seen in the male and female 

 photographed in fig. 3, the latter being much the darker of 

 the two. 



In regard to the original formation of the genital pouches, 

 nothing can be offered in the present paper, since the deve- 

 lopment is not followed that far. Hubrecht considers (26) 

 that they arise as invaginations of the ectoderm in Lineus, 

 but Burger's comment seems far more probable. 



