116 Animal Micrology 



II. Sections. 



48 hours, transverse, sagittal, and frontal. 



36 " " " 



30 " " " 



24 " 



18 " 



10 " " 



72 " " 



96 " " " 



The number of embryos needed for the above preparations is as follows: 

 5 embryos of 48 hours (27-29 somites). 

 4 " " 36 " (15-18 " ). 



3 "30 " (10-14 " ). 



4 " " 24 " ( 4-6 " ). 



2 " " 18 " 

 1 " 12 " 

 1 " " 60 " 



3 " " 64-72 hours (cervical flexure formed). 

 3 " " 96 hours. 



2. In Measuring the Length of Embryos some embryologists (e. g., 

 Minot) measure the greatest length of the embryo along a straight line 

 (limbs not included) when the embryo is in its normal attitude; conse- 

 quently in some early stages where the embryo is greatly flexed the 

 neck-bend would be the point to which to measure instead of the tip of 

 the head, because it is the most anterior region; in stages where the 

 embryo is straight, the head would be included. Other embryologists 

 (e. g., His and German authors in general) make use exclusively of the 

 so-called "neck-length ;" that is, the distance in a straight line between 

 the neck-bend and the caudal-bend. In this volume the full length 

 measurements are employed unless otherwise specified. 



3. To Mark Anterior and Posterior Ends of Young Chick Embryos in 

 blastoderms which still have a homogeneous aspect, Duval's osmic acid 

 method is very useful. With a strip of paper 5 mm. wide by 50 mm. 

 long a triangular bottomless box with narrow base is constructed. This 

 is placed on the yolk inclosing the blastoderm in such a position that the 

 base of the triangle corresponds to what will be the anterior region of 

 the embryo (for orientation of embryo in the egg see step 1 of the prac- 

 tical exercise). Press the box down against the yolk and fill it with a 

 0.3 per cent, aqueous solution of osmic acid. In a short time the prepa- 

 ration begins to darken and the osmic acid should be removed. The 

 blastoderm may then be removed in the ordinary manner and fixed as 

 desired (Duval used chromic acid for fixing). However, it is very dim*- 



