322 F. L. LANDACRE 



MATERIAL AND METHOD 



In a type which develops so rapidly as Ameiurus, there being 

 only five days between fertilization and hatching, the age seems 

 to be a better way to designate a series than the body length. 

 Series must be taken at close intervals and, while individuals of 

 a given age may not vary by an appreciable amount in length, 

 there are frequently found in the same series quite perceptible 

 variations in the degree of differentiation of the ganglia and sense 

 organs, so that while the age is not an absolutely accurate method 

 of designating a series since the growth varies with temperature, I 

 have used this on account of the difficulty of separating embryos 

 by their length. 



As to method, I have followed consistently the plan of locating 

 definite ganglia in older series after they were well defined and 

 tracing these back to the earliest recognizable stages. This plan 

 seems to be absolutely necessary, since only in a few cases do the 

 definitive ganglia use all of the material from which they are formed 

 and in some cases, particularly the general cutaneous ganglia, 

 only a very small portion of the mass from which the ganglion is 

 formed finally enters into the composition of the ganglion. Some 

 confusion seems to have arisen, especially in the earlier work on 

 nerves and ganglia, from taking it for granted that all of the 

 material from which a ganglion forms enters into a ganglion. 



The material on which this work was done is in part, the same 

 as that used in a former paper by the author (Landacre, '07) and 

 consists of series of Ameiurus melas of which the absolute age 

 is known since the process of oviposition was observed; and 

 in part, particularly the early stages, the work was done on a large 

 number of graded series of Ameiurus nebulosus of which the rela- 

 tive age was known but the absolute age was not known. The series 

 of Ameiurus melas are indicated by their ages. Of the nebulosus 

 material nine stages were used of which the last two series, VIII 

 and IX, are quite similar to each other and correspond closely 

 to the forty-nine hour series of A. melas. Series I seems to be 

 about twenty-four hours old compared with a similar stage of A. 

 melas. The remaining series II to VII inclusive are all younger 



