PRATT: IMAGINAL DISCS. 
251 
show that Graber’s conclusions were in great part false. I shall 
show that in the Brachycera the imaginal discs of the head and 
thorax all arise in the embryo first as thickenings of the ectodermic 
body-wall, and not in the form of endogenic plates; that these 
thickenings soon invaginate and form simple pockets in the ecto¬ 
derm. I shall show that the cephalic invaginations are formed 
before the involution of the embryonic head characteristic of 
brachycerous development, and that as a result of this involution, 
whereby the so-called pharynx of the larva is formed from the walls 
of the embryonic head, the discs come to form diverticula of the 
pharynx, in which condition they are found during the entire larval 
and the first part of the pupal period. It will also be shown that 
the thoracic invaginations finally become separated from the embry¬ 
onic ectoderm, and form the thoracic discs as we find them in the 
larva; also that the external genital organs take their origin in 
the form of imaginal discs in the same way as do the thoracic 
extremities. 
The Earlier Developmental Stages of Melophagus. 
1. The Development of the Egg to the Completion of the Blasto¬ 
derm . — The formation of the blastoderm in the embryos of brachy¬ 
cerous Diptera has been observed and described by the following 
authors, — in Musca vomitoria by Weismann (’64, ’82), Kowal- 
pvsky (’86), Blochmann (’87), Henking (’88), Yoeltzkow (’89), and 
Graber (’89) , and in Lucilia by Graber (’89). The accounts differ 
considerably from one another, although they are descriptions of the 
same process and all the authors mentioned are well-known, expe¬ 
rienced, and competent observers. On this account, the details of 
blastoderm formation in Melophagus will be a useful contribution 
to the subject. 
Melophagus is not, however, a favorable object for the study 
of the earlier stages of insect development, because of the impossi¬ 
bility of obtaining considerable numbers of its eggs and of deter¬ 
mining the age of the embryos within them. The female insect 
produces only one egg at a time and at intervals of several 
weeks; each egg must be dissected from the maternal uterus, 
where development goes on, and the age of the embryo can be 
ascertained only after the egg lias been properly stained, and then 
