The Early Development of the summer egg of 
a Cladoceran (Simocephalus vetulus). 
By 
d 
H. Graham Cannon, B.A., 
Demonstrator in Zoology, Imperial College of Science, South Kensington. 
With Plate 25 and 1 Text-figure. 
CONTENTS. 
PAGE 
1. INTRODUCTION : ; - : . : : 5 eGo 
2. MrerHop 5 5 F ‘ ; : ; F j 5 URIS 
3. EGG-LAYING . ; ‘ : : ; ‘ 3 : - 630 
4, CLEAVAGE . ‘ 3 p : 3 : : : a) ail 
5. FORMATION OF THE GERM-LAYERS . ; ‘ : 4 5 (832 
6. DiscussION . P : ; : : : : : . 636 
7. SUMMARY . : ‘ ; : : : : ‘ . 640 
8. BIBLIOGRAPHY : ; j : : 2 : P . 641 
9. EXPLANATION OF PLATE : 3 3 : : ‘ . 642 
1. INTRODUCTION. 
Tus paper deals chiefly with the method of germ-layer 
formation in the parthenogenetically-produced summer eggs 
of Simocephalus vetulus. A considerable amount of 
work has been done on the development of the eggs of various 
Cladocera, and a complete summary of this work is to be 
found in Vellmer’s paper (14) on the development of winter 
eges of Cladocera. In the same year that this paper appeared 
Kiithn (8) described very fully the development of the summer 
egg of Polyphemus pediculus, and here again is given 
a résumé of the work that has been done on Cladoceran 
development. This paper also reviews, in this connexion, 
