88 THE CHANGES FROM STAGE' A TO F. 



the somite itself, it becomes much reduced in size in Stage e, 

 being encroached upon by the rapidly growing cerebral rudi- 

 ments (Plate VII, fig. 22 a). In Stage f, in consequence of 

 the same process, combined with the great development of the 

 cerebral grooves, the first somite becomes much flattened out 

 (Plate VIII, fig. 33). It lies immediately over the white matter 

 of the posterior lobes of the brain, and becomes divided into 

 two parts posteriorly, viz. an external portion placed close 

 to and immediately dorsal to the eye, and an internal or median 

 portion (Plate IX, fig. 51, S. 1). The external portion, the 

 walls of which are thicker than those of the median, is 

 continuous with the rudimentary nephridial tube, of which 

 indeed it forms the dorsal end. 



The first somite, therefore, behaves exactly as do the posterior 

 somites. It becomes divided into two parts — a median part 

 and a lateral part. The latter sends out a ventral diverticulum, 

 which hugs the outer side of the nerve-cord and fuses with, if 

 it does not open through, the ventral ectoderm. 



Kennel has seen the rudimentary nephridium, but he has 

 not appreciated its significance or made out its exact relations 

 to the lateral division of the somite on the one hand and the 

 ventral ectoderm on the other. I should mention that by the 

 time the lateral division of the somite has completely sepa- 

 rated from the median, the nephridial tube has nearly vanished. 

 This fact may account for Kennel's omission to notice the con- 

 nection between the two structures. The suggestion by the 

 same author (No. 30, p. 49) that the eye may possibly be 

 derived from the ectodermal part of the lost nephridium of 

 the first somite, loses, after the discovery of the actual nephri- 

 dium, any plausibility which it might at first sight have 

 appeared to possess. It is obvious that the nephridium which 

 possesses a rudiment of an external opening behind and ven- 

 tral to the eye, and within the lip, can have nothing to do with 

 the eye which is derived from the side of the cerebral rudiment. 



The Somites of the Second Pair (somites of the segment of 

 the jaws) come to occupy in Stage b their permanent position 

 at the sides of the mouth. In Stage d (the eudoderm has 



