248 BERTRAM G. SMITH. 



large number of slides are required. For the main purposes of 

 this investigation the differentiation obtained by this method, 

 used with care, is entirely adequate, but as a precaution against 

 possible inaccuracy a parallel series was stained with Heidenhain's 

 iron haematoxylin, which as might be expected gave better 

 preparations, and is especially useful for the study of chromosomal 

 vesicles. 



All the figures excepting those of Plate IX. were outlined by 

 means of a camera lucida and drawn with a magnification of 

 about 600 diameters; in the process of reproduction they were 

 reduced one third, and thus appear with a magnification of about 

 400 diameters. In all drawings of horizontal sections the orienta- 

 tion with reference to the plane of first cleavage is the same, and 

 conforms to the diagrams in Plate IX. 



In most cases the double nature of the cleavage nuclei appears 

 to best advantage in horizontal sections, and might readily be 

 overlooked were observations confined to vertical sections. It 

 seems likely that most investigators studying the amphibian 

 egg have confined their observations to vertical sections, since 

 these give a better idea of the gross structure; and this circum- 

 stance leads us to suspect that conditions such as are here 

 described for Cryptobranchtts may occur in other amphibians 

 and need only to be revealed by the study of horizontal sections. 



OBSERVATIONS. 



The early history of the fertilization stage of Cryptobranchus 

 allegheniensis has been given in a previous paper (Smith, 1912, 

 I.). The superficial cleavage pattern has been described in 

 another contribution (Smith, 1912, II.). 



At the time of the meeting of the germ-nuclei it is usually 

 impossible to tell with certainty which is the egg-nucleus and 

 which is the sperm-nucleus, but this circumstance does not 

 affect the validity of the results recorded in this paper. The 

 germ-nuclei do not fuse, but come to lie side by side with nuclear 

 membranes intact (Figs. 1-4). During the long resting stage 

 that precedes the formation of the first cleavage spindle, each 

 germ-nucleus maintains strict individuality: there is close asso- 



