222 HENRY C. TRACY 



cult, but perimeningeal will apply to all this tissue except that 

 which is obviously subcutaneous. 



The term 'marginal membrane' (used by Streeter for the 

 membrane limiting the developing periotic spaces of the human 

 embryo) seems appropriate for the incomplete connective-tissue 

 walls found around some of these spaces. 



THE GENERAL HISTOLOGICAL STRUCTURE OF THE SWIMBLADDER 

 AND THE PRECOELOMIC DIVERTICULUM 



The wall of the swimbladder is composed of a lining mucosa 

 (tunica interna) and an outer strong connective-tissue layer 

 (tunica externa). A thin layer of loose connective tissue (sub- 

 mucosa) joins these two layers. The histological details of 

 these layers in the Clupeoids have not been investigated com- 

 pletely. The epithelium in most parts of the organ is simple 

 squamous; in some places, however, and particularly in the 

 pneumatic duct, it is cuboidal or columnar, and in Alosa sapi- 

 dissima is thrown up in complicated folds which possibly form 

 a primitive type of 'red gland.' The tunica propria of the 

 mucosa seems to consist of a delicate connective or reticular 

 tissue, with a network of fine elastic fibers in a layer under the 

 epithelium; in the outer part of the mucosa is a second layer of 

 elastic tissue. In Stolephorus a thick band of circular smooth 

 muscle occurs in the tunica propria at the constriction between 

 the two parts of the swimbladder, in the walls of the pneumatic 

 duct, and at the entrance of the precoelomic diverticulum into 

 the cartilage tube. 



The tunica externa is much thinner in the Clupeoid swim- 

 bladder than in that of many other teleosts. It is composed 

 almost wholly of coarse bands of connective tissue running cir- 

 cularly in a compact layer around the organ. The only elastic 

 elements shown in my sections are fine fibers running in a radial 

 direction through this layer. 



The precoelomic diverticulum of the swimbladder is formed 

 in the embryo by an anterior outgrowth from the forward end 

 of the anlage of the primitive swimbladder which pushes cephalad 

 into the cranial cavity. As might be expected from the manner 



