A in Jhvv CUTANEOUS PARASITE, THE ENTOZOON FOLLICULORUM. 
315 
twice their breadth, but, as they progressively enlarge, they undergo a correspond- 
ing alteration in their form ; they become elongated, they swell towards the centre, 
and one end acquires a slight predominance in bulk over the other. This change in 
the form of the ovum converts it into an embryo ; in one position the embryo is still 
oval, the one extremity being somewhat larger than the other, but as soon as it rolls 
over on its side the prominence of the middle part is perceptible. At this period 
the tail is shorter than the bulky part, and the latter scarcely presents any trace of 
division into the two lobe-like protrusions that are subsequently formed. As the 
embryo progresses in size the lobes become more apparent, and, seen from the front, 
the entire body is not unlike an ace of clubs with an elongated stalk. The next 
advance in growth is indicated by a further increase in length of the tail portion, 
and a more equable enlargement of the rest, so that the subdivision of the convex 
mass, although still apparent, is less manifest than it had been previously. 
The form of embryo just described was that of the specimen which measured yyy 
of an inch in length. Lying near to this in the same mass of sebaceous substance 
was another, a very little longer, being only yy-g- of an inch in length, but different in 
proportions. It was divisible into a head, a thorax, and a tail-like abdomen. The 
head was a rounded portion of the mass, distinguished from the rest by a slight con- 
striction, but presenting no distinction of parts. The thorax was oval in shape, 
largest in the middle, and diminishing towards the ends ; and the abdomen was 
cylindrical, much less in diameter than the thorax, and uniform in size. The animal 
appeared to be composed internally of an assemblage of cells enclosed in a thin mem- 
brane. The cells were all of the same size, or differed but little in this respect, and 
there was no trace of markings upon any part of the tegumentary covering. There 
was one character however in this specimen which distinguishes it at once from the 
embryonic form, and which has induced me to regard it as the first stage of develop- 
ment of the young animal. This character is the presence of three pairs of legs, 
which are extremely short, and look like conical prolongations of the substance of 
the animal. In a more advanced stage the divisions of the body become more di- 
stinctly defined, and begin to assume the characters by which they are approximated 
to the perfect animal. The head, for instance, is seen to be composed of two oval- 
shaped lobes, and a conical central piece, projected at the extremity of the body ; the 
thorax still retains its oval form, but is broader behind than in front, and the abdomen 
tapers gradually backwards to a rounded point. Other indications of the immature 
state in more advanced stages are, — the absence of cephalic palpi ; the shortness and 
straitness of the thorax ; the absence or small size of the phalanges ; the great length 
of the cylindriform and tapering abdomen ; the slightness of the thoracic and abdo- 
minal markings ; and the confused and cellular structure of the interior. I have 
found the length of the young animal while possessing only three pairs of legs, to 
range between y^y and yy of an inch, while in breadth its measurements were from 
8lT3 to -giro. After the attainment of four pairs of legs, its length ranged between 
