356 F. L. LANDACRE 



At the time of the appearance of the common primordium of 

 the third and fourth organs the postauditory placode lies about 

 five sections posterior to it and can be distinguished easily from 

 the primordium by the fact that the placode still retains its char- 

 acteristic appearance, having elongated radially arranged cells, 

 while the primordium bears no resemblance to a lateral line organ, 

 being merely a thickening of the epidermis. Here, as in all other 

 lateral line organs, one must first locate the organ after it is fully 

 formed and then read back in series and locate the primordium. 

 This can be done first by counting sections and second by using 

 convenient land-marks in the body. This is necessary because 

 none of the lateral line organs at first have any resemblance to 

 the adult organs. 



In an embryo of 99 hours (A. melas) while the postauditory 

 placode is still recognizable as a thickening of the epidermis there 

 have appeared at least two lateral line organs posterior to it. 

 These two organs can be traced continuously in my older sections 

 and lie posterior to the large branch of the Xth nerve, while the 

 placode lies anterior to it. Owing to the fact that structures fre- 

 quently do not develop uniformily even when a series of graded 

 ages is used, it is not possible to make definite statements in regard 

 to the last stage of the postauditory placode. Of its moving back 

 from the vesicle and of its gradual reduction I am quite sure. In 

 an embryo of ninety-nine hours (A. melas) while the third and 

 fourth lateral line organs are still contained in the common 

 primordium the placode is recognizable and back of this are two 

 primordia of lateral line organs. In an embryo of one hundred 

 and five hours the placode is present but no organs lie posterior 

 to it. In an embryo of one hundred and thirteen hours the two 

 lateral line organs previously mentioned as lying posterior to 

 the placode are present in practically the same position as in 

 the embryo of ninety-nine hours, having moved slightly away 

 from the vesicle while the sections occupied by the placode in the 

 preceding series are vacant and remain vacant in later series. 

 These two organs persist in my later series and I infer that the 

 postauditory placode disappears posterior to the position of the 



