252 J. THOS. PATTERSON. 



ing, can be used for experimentation ; for by this time the shell is 

 firm enough to permit handling without injury to the blastoderm. 



In dealing with this material it was necessary to use special 

 technique both for fixing and orientation. The picro-sulphuric- 

 acetic mixtures have been found to be vastly superior to all other 

 reagents. Of these mixtures the most successful is 92 parts of 

 Kleinenberg's strong picro-sulphuric plus 8 parts of glacial 

 acetic. The whole yolk is immersed in this fluid for one hour 

 and is then treated with 70 per cent, alcohol for several hours, 

 after which it is placed in 80 per cent. At this point it is found 

 advisable to cut out a properly oriented wedge-shaped block of 

 yolk containing the blastoderm, with vitelline membrane still 

 attached. After completely washing out the picric acid, this 

 block is carried through the higher alcohols, cleared in cedar oil, 

 and embedded and sectioned in the usual way. 



For stages prior to the appearance of the primitive streak, such 

 a treatment necessitates a careful orientation of the blastoderm 

 before using the fixing fluid. Already a method for orienting 

 the chick blastoderm has been worked out. Thus a number of 

 investigators have shown that if a hen's egg be held in front of 

 the observer so that the blunt end is to his left and the pointed 

 end to his right; the posterior margin of the blastoderm will be 

 towards and the anterior away from him, and hence, when the 

 embryo appears, its head will be directed away from the ob- 

 server, with its long axis meeting the chalazal axis at right angles. 

 If a pigeon's egg be held in a similar position a different con- 

 dition is found. The posterior margin of the blastoderm, instead 

 of being directly in front of the observer, is forty-five degrees to 

 his left, and when the embryo arises, its long axis meets the short 

 axis of the egg at an angle of forty-five degrees (see Fig. 1). 



For some purposes iron hematoxylin has been of great value 

 as a stain, but for general use a modification of Delafield's haema- 

 toxylin is unsurpassed, especially for demonstrating the presence 

 of cell walls. 



Gastrulation. 



I stated above that it was necessary to investigate the period of 

 development that occurs before laying. A study of these early 

 stages naturally involves the question of the origin of the two 



