NATURAL HISTORY OF AMERICAN LOBSTER 
325 
the forked telson-plate of the larva and touches or overlaps the lip. In 3 weeks the 
conical eyestalks are most prominent; 8 to 9 pairs of appendages are present, and 
the telson overlaps the brain. The brownish black eye pigment of the retinal cells 
begins to appear in the fourth week as a thin crescent at the base of each lobe, and 
gradually extends in area until in 3 or 4 months time it forms the large, rounded eye 
spots, so conspicuous a mark from this time onward. A cuticle to be later absorbed 
surrounds every part of the embryo, and rudimentary setae are beginning to appear on 
the telson plate and antennae. 
Up to the fourth week internal changes, which we shall not attempt to describe, 
have led to the already complex foundations of the nervous and muscular systems, 
the heart, and alimentary tract. Of the latter the stomodaeum or oral invagination 
gives rise to a distinct pouch from the epithelial lining of which the cuticular coat of 
the mouth opening, esophagus, and stomach sac are derived. The proctodaeum, 
feo which the anal opening and lining of most of the intestine is due, is similarly formed 
through a median ingrowth of ectoderm near the posterior end of what becomes the 
thoracic abdominal fold. The cuticular lining of the intestine when formed, like that 
of the stomodaeum, is continuous with the outer skin and must be shed at every sub- 
sequent molt. The proctodaeal invagination is at first solid or nearly so and is not 
sharply bounded from the yolk, which with its inclosed cells distinguished as hvpoblast, 
represents the embryonic section of the digestive tract, called the mesenteron, and 
gives rise to the gastric glands and to the epithelial wall of a small section of the tract 
into which they open. The walls of the mesenteron become continuous with those 
of the proctodaeum and are gradually extended forward on all sides until the entire 
yolk mass of the egg is inclosed within the folds of the paired gastric glands and forward 
division of the intestine. At a later period of embryonic life the screen which separates 
the stomodaeum from the yolk is absorbed and its walls unite with those of the mesenteron. 
At the time of hatching the residue of the yolk lies in the folded walls of the lobulated 
gastric glands, from which it is finally absorbed. This residual yolk sometimes appears 
to pass to the masticatory stomach, but if this ever happens it must be due to secondary 
displacement, as will be readily understood from the relation of the yolk to the mesenteron 
just described. The functions of digestion and absorption, which the gastric glands 
or liver display on a large scale throughout the embryonic period, are retained in adult 
life as already noticed. (See p. 249.) 
The intestine in the higher Crustacea, excepting only its terminal portion, is com- 
monly described as arising from the endodermal or hypoblastic wall of the midgut, 
or mesenteron, but this is certainly not the case in the lobster, which sheds an intestinal 
cuticle during its pelagic stages. A median longitudinal section through the body 
of the larva at the time of hatching shows a distinct cuticle passing forward along 
nearly the entire length of the intestinal tube, and finally shading off and disappearing 
opposite the gastric glands. The epithelial lining of the intestine is therefore almost 
wholly of ectodermic origin and continuous with the epithelium of the skin, a conclusion 
which embryological study fully supports. Apparently in the adult animal the cuticular 
