atlas 4-— 1 



Fig. 15 Cross section through the extreme posterior end of the occipital con- 

 dyle of an embryo slightly younger than that represented in the preceding figures, 

 showing the chorda dorsalis (c.d.) incompletely surrounded by the occipital con- 

 dyle which is here hypochordal in position. X 15. 



Figs. 16 to 19 A series of four cross sections through the ethmoidal region of 

 an embryo having a carapace 13.5 mm. long. The series is from anterior to pos- 

 terior. In figure 16, on the right side, a cartilaginous nodule is Ij'ing in contact 

 by its median surface with the pila supraglandularis (p.sg.). In figure 17 the pila 

 supraglandularis has extended ventrally and fused with the lamina terminalis 

 anterior {l.t.a.). The thickening of the paries nasi to form a concha is greatest 

 in figure 19. X 15. 



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