igS CLAXKE. [Vol. V. 



thinner at the point of apparent connection. It is only a thin 

 area, however ; there is no breaking through of the outer part 

 of the wall, although it has that appearance in the very trans- 

 parent living specimen. Neither of the clefts are yet open to 

 the exterior. Fig. 34 is also from life, the precise age un- 

 known, as the eggs had evidently been in the nest a long time 

 — two weeks or more — when found. The caudal region is here 

 seen to be free from the yolk ; the mesoblastic somites extend 

 to the end of the tail, and anteriorly nearly to the ear. A series 

 of similarly appearing structures to the somites in the mesoblast 

 are continued forward as far as the front end of the alimentary 

 tract : they are apparently in the central nervous system, and 

 have been described in several other forms as neuromeres. 

 They resemble the numerous wrinkles 1 "which appear in that 

 portion of the medullary canal of the chick which lies imme- 

 diately behind the first cerebral vesicle." These structures 

 have recently acquired more importance through the labors of 

 McClure (12). The peculiar appearance of the two pharyngeal 

 clefts is partly due to the great transparency of certain parts of 

 the embryo at this stage, when examined in the fresh state, 

 and it is partly owing to the greater thickness of the epithe- 

 lium lining the ventral portion of the clefts. After preserva- 

 tion in picric acid, or still better for external features in chromic 

 acid, the outlines become much more distinct and clear. The 

 first and second pharyngeal clefts at this stage are in process 

 of formation, and their structure is given in Fig. 40. The 

 drawing was made from a stained and mounted embryo, viewed 

 by transmitted light. The first cleft is nearly completed and 

 is connected by an internal groove with the rudiment of the 

 second : this rudiment is the ventral portion of the cleft. 



The clefts have increased to three with the rudiment of the 

 fourth in the somewhat later stage of Fig. 35. 



And here the fourth cleft has a similar connection with the 

 third, which the second has to the first in Fig. 33. The embryo 

 shown in Fig. 41 is seen when viewed by transmitted light to 

 have three clefts formed, and in connection with the third there 

 is an internal groove which extends backwards, and from which 

 there appears to be forming the rudiments of the fourth and 

 fifth clefts. The fourth and fifth are small and rudimentary, 



1 Foster and Balfour, Elements of Embryology, p. ioo. 



