All the variations observed in the adult cæcum are derived from these two fœtal types by a subsequent and usually asymmetrical enlargement and dilatation of the pouch.

We can consider the derivatives of each form separately.

I. Adult Cæca Derived From Type I. (schema, [Fig. 509], Ia, [Fig. 510]).

1. Further development leads to an enlargement of the cæcal pouch and a sharper demarcation between the same and the appendix. The resulting cæcum is symmetrical, with equally developed lateral sacculi, between which the termination of the longitudinal muscular bands and the root of the appendix is situated (schema, [Fig. 509], Ib).

Fig. 515.—Human fœtus at term. Cæcum and ileo-colic junction; ventral view. (Columbia University Museum, No. 1510.)   Fig. 516.—Human fœtus at term. Cæcum and ileo-colic junction; ventral view. (Columbia University Museum, No. 1548.)

In Figs. 515 and 516 two infantile cæca are shown which illustrate this form. The narrow and pointed apex of the fœtal conical cæcum is replaced by the capacious pouch which is differentiated sharply from the appendix. Among the anthropoid apes the same type is seen in the cæcum of the gibbon ([Figs. 455] and [456]), and of the young chimpanzee shown in [Fig. 460].

2. An increased development of the cæcal pouch in the adult leads to the protrusion caudad of two symmetrical sacculations on each side of the root of the appendix which appears between them. The original apex of the cæcal pouch is still marked by the implantation of the appendix and by the termination of the longitudinal muscular bands, but the lowest level of the pouch is found on each side of this point at the fundus of the secondary lateral sacculi (schema, [Fig. 509], Ic). Treves, to whom belongs the credit of first accurately describing and classifying the forms of the adult cæcum based on the development, found this type in three of a series of 100 cases examined.

Fig. 517.—Adult human cæcum and ileo-colic junction. (Columbia University, Study Collection.)   Fig. 518.—Adult human cæcum and ileo-colic junction. (Columbia University Museum, No. 234.)

Figs. 517 and 518 illustrate this form of the pouch, which, in our experience, is frequently associated with the retro-cæcal erect position of the appendix (cf. infra, [p. 251]). [Fig. 472] shows this type in the adult with pendent appendix.

II. Adult Cæca Derived from Type II. (schema, [Figs. 509] and [511]).—From this more commonly observed type of fœtal cæcum the following adult forms are developed: