THE INTEGUMENT AND THE EXQSKELETON 109 



Jsually also a gland cell, specialized as it were in this direc- 

 ;ion, secretes these products in greater abundance than in the 

 ise of other cells. As a single cell may thus have all the attri- 

 butes of a gland, the simplest glands are composed of but one 

 such elementary unit and are 'unicellular. Such simple glands 

 are of extensive occurrence among animals and are generally 

 used where a surface is to be kept uniformly moistened with 

 some secretion, as a protection against water or air, and where 

 there is no special auxiliary structure, like the eyelids of land 

 vertebrates, to insure an even distribution. The majority of 

 glands, however, are multicellular and represent various solu- 

 tions of the problems of how to increase the physiological 

 efficiency within a definite space, i. e., how to increase the 

 effective secreting surface without increasing the mass. The 

 diagrams in Fig. 29 represent various solutions of this prob- 

 lem, as well as varying degrees of physiological efficiency, the 

 most complex form being in general the most successful. Be- 

 ginning with single cells opening upon a free surface it is 

 evident that the efficiency increases with the number of gland 

 cells in a given space, the limit of this type being reached 

 when all the cells have become thus employed. If, however, 

 the problem allows the utilization of a certain amount of depth, 

 the efficiency may become much increased by folding or 

 invaginating portions of the original surface below the general 

 level, either in the form of tubules (b) or flask-shaped glo- 

 bules, acini (c). Each of these primary types may become 

 still further complicated in several ways. The tubular form 

 may become convoluted (d) or branched (e), and the acinous 

 form may develop secondary acini (f). Through a slight 

 cellular differentiation the cells nearest the outlet of the gland 

 may become flattened and form a non-secreting duct through 

 which may pass the fluid manufactured in the secretory por- 

 tion. This principle may be extended to the secondary acini, 

 and when these latter become profusely multiplied, the result 

 is a definitely localized and very effective organ, as in (g). 

 These varied forms are not sharply defined, and even the 

 fundamental types of tubular and acinous glands may grade 



