350 Mr. R. Assheton. An Experimental Examination 



(1) To test by actual experiment Duval's* theory of the formation 



of the primitive streak. 



(2) To try and determine experimentally whether the whole or 



only part of the actual embryo is developed by the activity 

 of the primitive streak. And further, if only a part, to 

 determine its limits. 



With regard to the first question it may be remarked that Duval's 

 account is generally accepted, although perhaps greater stress is laid 

 upon it by foreign and American writers than by embryologists in 

 this country. 



According to Duval's account, there is in the freshly laid and unin- 

 cubated egg a groove which separates the blastoderm from the yolk. 

 The groove, he says, is broader and more conspicuous at the posterior 

 margin than at any other point.' This he compares to the anus of 

 Rusconi or blastopore of the segmenting, frog's egg. 



During the first few hours of incubation the edge of the blastoderm 

 is said to advance over the yolk at every point except at this most 

 posterior margin bounding the groove, which he regards as equiva- 

 lent to the frog's blastopore. At this spot there is no advance. The 

 portions of the edge of the blastoderm adjoining this part swing 

 round to meet each other in the middle line, and eventually fuse 

 and form what Duval calls the " plaque axiale." 



This structure is in reality the primitive streak, and, according to 

 Duval, it becomes visible as such during about the tenth to fifteenth 

 hours of incubation by reason of the subsequent hollowing out of 

 the subjacent yolk by the extension backwards of the sub-germinal 

 cavity. 



Such a mode of growth would be very extraordinary and interest- 

 ing if true, and would be very acceptable to those who believe that 

 the growth in length of the vertebrate embryo is caused by a concre- 

 scence of two at first separated germinal rims. 



Naturally this account of the formation of the primitive streak as 

 given by Duval is frequently quoted by the many adherents to the 

 concrescence theory. . 



, During the last few years experimental methods have been intro- 

 duced much more freely into investigations of animal development. 

 Foremost amongst the workers upon these lines is Dr. Wilhelm Boux, 

 who experimented by destroying certain cells of the segmenting eggs 

 of frogs, and noting the result after some days of development. He 

 has been followed in similar work by Morgan and Time Tsuda and 

 others. 



The eggs of frogs have been the object of experiment of a different 



* " De la Formation du Blastoderme dans 1'QEuf d'Oiseau," ' Annales des 

 Sciences Naturelles, Zoologie,' vol. 18. 



