EMBRYOLOGY OF THE ISOPOD CRUSTACEA. 



J. PLAYFAIR MCMURRICH. 



CONTENTS. 



PAGE 



Part I. The Phenomena of Impregnation and the Formation of 



THE Egg-membranes in Jaera 66 



1. The process of impregnation in Jaera 66 



2. The formation of the egg-menih-anes in Jaera 68 



Part II. The Segmentation and Formation of the Germ-layers 73 



1. The segmentation and formation of the gei'm-layers in Jaera.. 73 



2. The segmeiitation and formation of the germ-layers in Asellns 



comtmcnis 88 



3. The segmentation and formation of the gerjti-layers in Porcellio 



and Armadillidium 95 



4. General consideration of the segmentation 107 



Part III. The Later Development of the Germ-Layers 115 



1. The later history of the Mesoderm 115 



2. The formation of the digestive tract and the later history of the 



vitcllophags 124 



3. General considerations on the formation of the germ-layers in 



the Crustacea 128 



Part IV. Notes on the Development of Certain Organs 137 



The observations recorded in the following pages were com- 

 menced in the summer of i8go at the Marine Biological Labora- 

 tory, Woods Holl, Mass. The object in view was to follow 

 out for the Crustacea the cytogenetic development after the 

 manner which has yielded such important results in the case of 

 Clcpsinc (Whitman) and Nereis (Wilson), and after examin- 

 ing some early stages of several different species of Decapods, 

 Isopods, and Amphipods, I finally chose for my purpose the 

 ova of Jaera marina (Fabr.) Mobius, which seemed to present 

 several advantages, notably in the early differentiation of the 

 germ-layers. In addition, at that time the embryology of the 

 Lsopods had received, on the whole, less attention than it was 

 entitled to, and considerable differences seemed to exist in the 

 modes of segmentation of the forms which had come under ob- 



