No. 13.— The Embryonic History of Imagined Discs in Melo- 
phagus ovinus X., together with an Account of the Earlier 
Stages in the Development of the Insect} 
By H. S. Pratt, Ph. D. 
With seven plates. 
CONTENTS. 
Introduction. 
Historical: 
1. Imaginal Discs in the Larva and Pupa . ... 
2. Imaginal Discs in the Embryo. 
The Earlier Developmental Stages of Melophagus : 
1. The Development of the Egg to the Completion of the Blasto¬ 
derm . 
2. The Formation of the Mesoderm and of the Proctodeum and 
Stomodeum. 
The Origin of the Imaginal Discs: 
1. The Cephalic Discs ... 
a. The Early Development of these Discs . 
b. The Involution of the Head ....... 
2. The Thoracic Discs ......... 
3. The Discs of the External Genitalia . . . . 
4. The Discs of the Internal Organs. 
Methods. 
Bibliography ... 
Introduction. 
In 1897 I published in “ Psyche ” a preliminary account of the 
origin and early development in Melophagus ovinus of those funda¬ 
ments of the imaginal head, wings, and appendages, which may be 
present in the larva of holometabolic insects and are known as 
imaginal discs. The present paper is an extended account of the 
same matter. 
Melophagus belongs to the small group of brachycerous Diptera 
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1 Contributions from the Zoological Laboratory of the Museum of Comparative Zoology 
at Harvard College, under the direction of E. L. Mark, Iso. 111. 
