THE RELATIONS OF SURFACE TO MASS. 21 



furaginations exist in much more varied form and play a principal part in 

 the differentiation of the animal body. They may be classified under four principal 

 heads: (i) dilatations; (2) diverticula; (3) glands; (4) vesicles. Dilatations have 

 considerable importance in embryology: the stomach, lungs, bladder, and uterus 

 arise as gradual dilatations of canals or tubes of originally nearly uniform diam- 

 eters. Diverticula^ in the sense of relatively large blind pouches, also form impor- 

 tant organs, such as the caecum and appendix vermiformis, or the gall-bladder; 

 these structures arise each as a blind outgrowth of a canal, the walls of which at 

 a certain point rapidly grow to form the pouch. Glands are, as first shown by 

 Johannes Miiller's classic researches, only small diverticula, which end blindly and 

 appear in an immense variety of modifications; the manifold types of glands are 

 discussed below in a separate paragraph; they constitute the largest class of organs 

 with which we have to deal. The glands are developed from epithelium and push 

 their way into the mesoderm upon which the epithelium rests, while in dilatations, 

 and in diverticula, .the epithelium and mesoderm expand together. Vesicles we call 

 those epithelial sacs which develop somewhat like glands by growing into the meso- 

 derm, but the mouth of the invagination closes by the coalescence of the epithelium, 

 thus shutting the cavity. The closed sac separates from the epithelium from which 

 it arose, and connective tissue grows between the two; the sac may then undergo 

 various modifications. The membranous labyrinth of the ear is developed from the 

 ectoderm in this way, as is also the lens of the eye. We might perhaps also class 

 the medullary canal under this head (cf . Chapter V) if we choose to consider it as 

 a vesicle so much lengthened that it has become a tube. 



Glands. A gland may be defined as a structure which produces material which 

 is discharged from the gland and used elsewhere to meet a physiological need. Ac- 

 cording to the nature of this material, we distinguish two fundamentally different types 

 of so-called glands. One of these we designate as the true glands, which produce 

 chemical substances which are thrown off from the cells producing them to constitute 

 the secretion of the gland, so that the cells themselves all remain in the gland. In 

 the second type the cells themselves are multiplied, so that the structure yields, as 

 it were, a crop of cells, which is removed from the site of origin and then utilized 

 physiologically. Glands of this type may be called cytogenic. Of the true glands 

 we may distinguish several sorts. The simplest kind consists of a single cell. Of 

 unicellular glands, the goblet cells are the most familiar type known in man. Most 

 true glands, however, comprise many cells and are -classed as multicellular. The 

 majority of these have an internal cavity, which may be simple or very complicated 

 in its form, but is always bounded by a layer of epithelium. They have in addi- 

 tion a canal, which leads from the cavity of the gland to an external opening, 

 and is called the duct. When the secretion is produced, the chemical substances 

 formed by the epithelial cells are discharged into the cavity of the gland and thence 

 flow through the duct to the outlet. In certain cases a remarkable modification 

 may occur by the obliteration of the duct, thus producing a so-called ductless epi- 



