582 F. L. LANDACRE AND A. C. CONGER 



does not trace the latter to the auditory vesicle or its anterior 

 or posterior extensions but finds them originating separately and 

 distinct from those structures. The auditory vesicle and its an- 

 terior and posterior extensions appear first and are followed by 

 the sensory lines along the course of which the lateral line organs 

 differentiate, or as in the case of Ameiurus, even by the appear- 

 ance of individual lateral line organs which are not preceded by 

 sensory lines, 



MATERIAL AND METHOD 



The material used in this investigation consists of series of 

 Lepidosteus osseus, taken from one lot of eggs at six-hour stages, 

 and fixed in Zenker's fluid. Although the interval is long, an 

 examination of a number of individuals shows sufficient varia- 

 tion in the degree of development to make the series practically 

 continuous. The sections were cut 6 ^ thick and stained in bulk 

 for twenty-four hours in Delafield's hematoxylin, one-sixth the 

 strength of stock solution. 



Table 1 shows the age, increment in hours, the length and in- 

 crement in length wherever possible. Stages X to XVII are 

 referred to in the discussion by age only, since in these stages the 

 embryos were not removed from the membranes before fixation 

 and consequently accurate measurements cannot be given. 



The method employed has been to locate the sensory lines in 

 an advanced stage (stage XXVI) and then trace these structures 

 back through development to their earliest recognizable stages. 

 Since it has been the purpose to find, if possible, the relation of 

 the preauditory placode (Landacre '10, 'branchial sense organ' of 

 Wilson and Beard) to the sensory lines, we have traced the de- 

 velopment of the preauditory placode from the point where it is 

 continuous with the auditory vesicle up to the last stage where it 

 can be positively identified. The history of the ectodermal 

 thickenings in the region where the endoderm of the hyoid gill 

 pocket approaches the epidermis have also been carefully studied, 

 since their size in this type renders them important factors in 

 this investigation. 



