218 
PROFESSOR HUXLEY ON THE ANATOMY 
of the series. Those already formed elongate, and new longitudinal hars are added, until 
the walls of the branchial sac assume their perfect form*. 
As I have explained, the sac-like alimentary tract originally ends in a conical point at 
that extremity which is opposite its oral end ; and this cone is connected with the external 
tunic. In subsequent stages the cone remains distinct, being directed at an obtuse angle 
to the rest of the hsemal wall of the pharynx, while the cellular bands which eventually 
render the endostyle so conspicuous, cease at its base. It, at first, communicates by its 
widely open base with the pharyngeal or branchial cavity ; but as development proceeds, 
it becomes narrower, in proportion to the endostyle, and at length is represented by that 
slender backward prolongation of the endostyle or 'endostylic cone' described at the 
commencement of this memoir and represented in fig. 14. 
The languets do not appear till development has advanced a long way ; in fact, in the 
very young buds there is no room for them, as almost all the space between the place of the 
commencement of the oesophagus and the place of the ganglion, is occupied by the aperture 
of communication between the prolongation of the endostylic cone and the pharynx. As 
growth proceeds, the distance between the ganglion and the oesophageal aperture gradu- 
ally increases, both absolutely and relatively, and in buds -g^th of an inch long, one or two 
small tubercles are visible, projecting from the hypopharyngeal band, between the oeso- 
phageal aperture and that of the canal which traverses the prolongation of the endostylic 
cone. These gradually increase in number, elongate, and assume their adult shape and 
size (figs. 24, 25). 
The figures will sufficiently explain the further changes of form undergone by the 
gastro- intestinal portion of the alimentary canal. 
The hepatic tubular system makes its appearance in such buds as that represented in 
fig. 22, as a minute diverticulum of the stomach, which elongates, applies itself to the 
intestine and ramifies over it. Krohn (I. c. p. 331) saw it originate in a similar manner 
in P hall us ia. 
The heart, similar to that of the adult in form and texture, is distinctly discernible in 
buds not more than ^jth of an inch long, attached, in its ordinary position, to the wall 
of the pharynx, just in front of the bend of the intestine, between it and the endostylic 
I have not been able to trace out the first condition of this organ and the changes 
which it undergoes in acquiring the state now described. 
cone. 
The renal organs are plainly visible in buds not more than -^th of an inch long as 
aggregations of clear, round, almost colourless corpuscles, between the atrial and the outer 
tunic. 
In describing the first stage of the bud (fig. 14), I have spoken of a thin layer of in- 
different tissue which passed from the end of the endostylic cone, or prolongation, into the 
generative blastema. In more advanced stages, this tissue forms a sort of hood 
und 
Krohn (I. e. pp. 324 and 327) states that the stigmata of the embryo Phallusia make their appearance as roi 
apertures ; but he affirms that new ones are added, not only in front and behind, but in the neural and haemal sic es 
of the first formed series. 
