462 R. M. PACE. 



from the cells of the internal sac (PI. 25, fig. 65). The signi- 

 ficance of its appearance will be discussed in a later section. 



The second oral ectodermic invagination is destined to 

 form an organ representing the pharynx; it occurs in front 

 of that from which the internal sac arises, and appears at a 

 slightly later period. The cells bounding its opening arc 

 large and high, their nuclei lie on the side nearest the open- 

 ino- and the cells themselves are much vacuolated. The 

 vacuoles (PL 25, figs. 62-64) in the walls of the pharynx are 

 filled with a substance similar in appearance to that which 

 has been described as occurring in the cells of the internal 

 sac (PI. 25, fig. 62), and, as in the latter case, this substance 

 also entirely disappears at a later stage (PI. 25, fig. 65). 

 Fi-om its mode of origin, the pharynx is to be regarded as a 

 true stomodfeum. 



The pyriform organ arises, at a somewhat later period 

 than the internal sac and the pharynx, as an oral ecto- 

 dermic invagination anterior to the latter organs. In PI. 25, 

 figs. 62, 63 a, the internal sac and the pharynx are shown 

 well developed, while at this stage the pyriform organ is 

 represented only by a slight invagination {Py.). 



The aboral organ, the "calotte" of French authors, 

 is at the stage figured in PI. 25, fig. 62, already visible as a 

 thickened mass {Ca.) of aboral ectoderm overlying the pharynx 

 and provided with numerous nuclei. From this organ a 

 delicate network of fibres and nuclei passes to the developing 

 pyriform organ (PI. 25, figs. 62, 63«). 



Organs of Mesendodermal Origin. — The three-fold 

 ectodermic invagination leading to the formation of the 

 internal sac, pharynx, and pyriform organ has the effect of 

 compressing the mesendoderm into a solid mass, which lies 

 in the posterior part of the larva with its anterior end over- 

 lying the inner end of the pharynx (PI. 25, fig. 62, Ed.). 

 This mesendodermal tract consists of a mass of yolk 

 spherules with scattered nuclei and it rapidly loses all trace 

 of definite cell structure. At a slightly later period, the 

 mesendodermic mass becomes hollowed out and forms what, 



