62 THE CHANGES FROM STAGE A TO ¥. 



stricted, but persists throughout life as the optic nerve. The 

 eyes of Peripatus are, therefore, as I stated in my original 

 paper (No. 50), cercbi'al eyes, and are from the very first in 

 connection with the cerebral ganglia.^ 



The tentacular nerves are to be regarded, from their method 

 of development, as forward prolongations of the cerebral 

 ganglia. 



In the outgrowths from the pre-oral lobes to form the 

 tentacles the cerebral thickenings, which extend dorsalwards 

 on to the anterior face of the former, participate in such a 

 manner as to form the ventral surface of the developing ten- 

 tacles (Plate VI, fig. 7). The deep rounded elements of these 

 forward tentacular continuations of the cerebral rudiments 

 eventually separate from the surface ectoderm, and become the 

 tentacular nerves. 



The later stage of the cerebral ganglia will be considered 

 subsequently. By the stage reached (Stage f) they are in 

 connection with the walls of the cerebral grooves. 



The latter are almost if not entirely constricted ofi" from the 

 surface, and are lined by the ectoderm which gave origin to 

 the cerebral ganglia. Kennel regards this invaginated layer 

 as the homologue of the ventral organs of the postoral region. 

 It difi'ers, however, from the latter in being invaginated. Not- 

 withstanding this, the view seems to me a plausible one. 



The cerebral ganglia of Peripatus resemble, therefore, in their 

 development and method of removal from the surface, the cen- 

 tral nervous system of the Vertebrata, and the cavity of the 

 ventral appendages of the adult brain corresponds^ with the 

 central canal of the latter. 



These cerebral grooves seem a fairly constant feature in 

 Tracheate embryos. They do not, so far as I know, persist into 

 the adult of any other Arthropod, but disappear in the course 

 of development. In these cases their walls are said to become 



' Kennel states that in the West Indian species the eye arises independently 

 of the brain, and secondarily enters into connection with it. I cannot, how- 

 ever, help thinking that he is mistaken on this point. 

 ^ By this, of course, I do not mean that the two structures are homologous. 



