264 CLA YPOLE. [Vou. XIV. 
mouth-parts are next in succession. On each side of these 
has appeared a ridge that passes backward along the embryo, 
the two folds enclosing the mandibles and maxillae. These 
folds start from just the region where the small intercalary 
appendages were seen earlier, but which have now disappeared. 
_Figs. 43, 46, and 47 show the process by which this change 
takes place, and leave no doubt that the folds as they finally 
appear are a development from the intercalary appendages. 
This sheath-like form of the extra mouth-part explains the 
well-known peculiar structure of the adult head. The adult 
mouth has always been described as deeply sunk into the 
head and appearing as a tube, out of the end of which the 
points of the mandibles and maxillae protrude. It can be 
readily seen that the labrum in front and these lateral folds 
make together a three-sided box in which the mouth-parts, two 
mandibles, and four maxillae are sheltered. 
In Fig. 43, where flexure has just begun, the thoracic appen- 
dages are visibly longer and more distinctively legs. The first 
abdominal segment bears a large pair of appendages that are 
ultimately modified to form the collophore, while on three of 
the succeeding abdominal segments there are also small appen- 
dages, those on the fourth segment being the largest. This 
condition is equally evident in Fig. 44, a slightly later stage. 
In Fig. 45 the conditions are still the same; the collophore is, 
however, almost hidden by the flexure of the body, and the 
terminal segment has elongated into two decided folds that 
surround the proctodaeum. The five single eye-spots have 
appeared on the sides of the head, and the precephalic organ is 
much reduced and shows the thread-like elongation of its cells. 
An interesting question is raised by a consideration of the 
folds that rise round the mouth. The simple structure of the 
adult mouth in these forms has been discussed by Fernald ('90) ; 
he describes it as being a pouch of considerable size, at the inner 
end of which are attached two pairs of jaws; these are entirely en- 
closed. He cannot, however, determine the exact homologies of 
the different parts. Hansen ('93) discusses the homologies of 
the mouth-parts of the Crustacea and Insecta by studies on 
Japyx, Campodea, and some of the Collembola. He finds it a 
