218 PROFESSOR HUXLEY ON THE ANATOMY 



of the series. Those ah-eady formed elongate, and new longitudinal bars are added, until 

 the walls of the branchial sac assume their perfect form*. 



As I haye 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 liEemal 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 

 Avidely 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 hj 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 alxsolutely and relatively, and in buds aVth 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 {l. c. p. 331) saw it originate in a similar manner 

 in Thallusia. 



The heart, similar to that of the adult in form and textiu'e, is distinctly discernible in 

 buds not more than -^(^ilx 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 

 cone. I have not been al)le to trace out the first condition of this organ and the changes 

 which it luidergoes in acquiring the state now described. 



The renal organs are plainly visible in buds not more than -g^oth of ^ii iiich 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- 

 difPerent 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 over the 



* Krohn {I. c. pp. 324 and .327) states that the stigmata of the embryo Phalhtsia make their appearance as round 

 apertures ; but he affirms that new ones are added, not only in front and behind, but in the neural and haemal sides 

 of the first formed series. 



