510 H. H. SWTNNE[iTON. 



shows any sign of these tlaey have ah'eady practically 

 assumed the condition shown iu figs. 1 and 6 (PI. 28),^ the 

 main difference being that the cartilaginous portions are not 

 yet so extensive. 



On either side of the ventrally flexed notochord (fig. 1, ch.) 

 the parachordals have become differentiated as long plate-like 

 tracts of skeletal tissue, in each of which there are two 

 distinct regions of chondrificatiou {p.ch., oc. e.) separated by 

 an intervening area of pro-cartilage, and corresponding to 

 Sewertzoff's mesotic and occipital sections (99, p. 310). In 

 embryos of the seventh day the whole length of each tract 

 is well chondrified, but these two regions may still be easily 

 distinguished. 



The mesotic section (p. ch.) extends nearly three quarters 

 of the length of the tract, and anteriorly its inner border is 

 separated from the notochord. From its outer border arises 

 a stout process which expands distally to form the auditory 

 capsule. At the same point a ridge starts Avhich divides the 

 section into two parts : an anterior which supports the brain, 

 and a posterior or otic portion which helps to support the 

 auditory organ. 



The occipital section {oc. e.) is much shorter, and shows 

 none of those signs of segmentation recorded by Sewertzoff 

 for Acanthias. Like the hinder part of the other section and 

 the intervening pro-cartilage, it lies in contact with the side 

 of the notochord. 



In the salmon, according to Stohr, the auditory capsule 

 arises first as a plate of pro-cartilage, under the auditory 

 vesicle, joined to the parachordal by fibrous tissue ; iu 

 Embryo III this plate becomes chondrified; in Embi-yo IV, 

 which corresponds to Parker's Stage V, it becomes connected 

 anteriorly with the end of the parachordal by a strip of 

 cartilage, and thus assumes the condition above described 

 for the stickleback. Since the larvte of Parker's Stage V 



^ 111 following I he description of this stage it should be borne in mind that 

 tlie model from which figs. 1 and 6 were taken was made from an embryo of 

 the prematurely released tyjie. 



