The Emhryology of Chlamydoselachus 599 



tures absent in the head of the 20'mm. specimen but found in the beginning stage in the 

 25'mm. embryo as seen in Text-figure 27b. 



As Figure 20, plate II, shows, this 32'mm. specimen differs markedly in the head 

 region from the 20'mm. embryo portrayed in Figure 18. The smoothly rounded fore- and 

 midbrain vesicles are separated from each other by a cleft. The olfactory organ is well 

 established, as is the eye which on the ventral side shows a trace of the optic cleft. The 

 mouth is still widely open but far less so than in the 20'mm. embryo (Figure 18, plate II). 

 The seven pairs of visceral clefts are open to the exterior and to the pharyngeal cavity, 

 the second being the largest and the seventh (the sixth branchial cleft) the smallest. The 

 first, the spiracular cleft, is almost closed off ventrally. 



Further comparison of the 20' and 32'mm. specimens shows that the crumpled gill' 

 slits of the 20'mm. embryo are in the 32'mm. head replaced by the more normal straight 

 ones. All the clefts (including the spiracular) have external filaments in the 32'mm. head. 

 It is interesting to note that the ridge immediately posterior to the eye in the 20'mm. 

 specimen together with the hinder region of the upper jaw have here developed into a very 

 prominent cheek'like process. The growth of this process apparently assists in the for' 

 mation of the hinder part of the upper jaw and of the cheek region while at the same time 

 superficially closing off the spiracle on its ventral side. It also assists in bringing the 

 posterior end of the upper jaw in closer proximity to the hyomandibular element of 

 the hyoid arch. 



Let us now contrast the lateral view of the 25'mm. head shown in Text'figure 27b 

 with that of the 32'mm. specimen portrayed in Figure 20, plate II. The 25'mm. head is 

 more smoothly rounded. The eye has a fissure at its hinder edge whereas the other head 

 has it in a ventral position. The mouth of the younger fish is more widely open. But 

 most unusual of all are the curious structures around the spiracular opening. These I do 

 not understand — possibly this specimen was abnormal. In any case these objects seem to 

 be forerunners of the cheek pieces of the 32'mm. specimen shown so prominently in 

 Figure 20, plate II. There is nothing unusual about the gill-arches and clefts of the 

 25'mm. fish save that the first is more widely open than that of the older specimen. 

 Behind the seventh cleft on each head and almost over the yolk'Stalk are the stubby pecto' 

 ral fins. Faint traces of the lateral line are seen on each specimen. 



Head in Ventral Aspect. — The drawing of the ventral aspect (Figure 21, plate II) 

 gives one a clearer idea of the morphology of the organs on the sides and lower surface of 

 the head. Note the head, blunt'pointed in this aspect, the prominent eyes, and also the 

 kidney'shaped outlines of the nasal pits. The cartilages of the lower jaw have united, but 

 between those of the upper there is still a gap. In the mouth behind this gap is a structure 

 which Nishikawa, by sectioning this head, identified as Rathke's pouch extending back 

 toward the infundibulum. In contrasting the 32'mm. head with that of the 25'mm. 

 specimen in ventral aspect, it is seen at once that the former is much older and more 

 "finished". Contrast the widely gaping round mouth of the younger fish, with its two 

 median fossae, with the far more normal mouth of the older specimen. No further comment 



