692 LIFE : OUTLINES OF GENERAL BIOLOGY 



abundant slime or mucus is due. But in the frog there are groups 

 of glandular cells forming minute pits in the skin ; in other words, the 

 superficial glands are no longer unicellular but multicellular. The 

 insinking of glandular epithelium forms narrow tubular glands in 

 some t^'pes, swollen alveolar glands in others, and there are further 

 complications, such as the abundant branching seen in the milk- 

 glands or mammary glands, which are at first sohd ingrowths, but 

 become secondarily hollowed out. From scattered glandular cells in 

 the earthworm's skin to the milk-glands of mammals is a long 

 stretch, but, whether simple or complex, there is always the same 

 essential glandular epithelium. Inside the gland-cell the living 

 matter manufactures a secretion in the form of granules which turn 

 into droplets, or of droplets from the first, and after it has become 

 "loaded" with the secretion there is a rapid "unloading" or dis- 

 charge, usually in response to a nerve-stimulus, but sometimes 

 because the blood has brought an exciting hormone. In some cases 

 a gland-cell makes one kind of secretion, like the mucus on the skin 

 of a fish ; in other cases there is a production of two secretions, as 

 at the upper or cardiac end of a frog's stomach ; or there may even 

 be three, as in the cells of the pancreas. While we have referred in 

 this outline to superficial glands in particular, most of the internal 

 glands are, to begin with, also derivable from glandular epitheHum. 

 Thus the pancreas, the most important digestive organ in the body, 

 is in the embryo a little pouch of the primitive food-canal, with a 

 lining continuous with the glandular epithelium of the duodenum — 

 the first part of the intestine. 



NERVOUS TISSUE.— In sponges, which illustrate the beginnings 

 of animal tissues, there are no nerve-cells, doubtless one of the 

 reasons why this type of animal does not seem to have led on to 

 any other. But in some sponges, e.g. Pachymatisma, the large 

 exhalant o]x^nings can be rapidly closed, e.g. when an inquisitive 

 worm inserts its head. The closure is due to the contraction of a ring 

 or sphincter of spindle-shaped cells, which might be called "neuro- 

 muscular" since they are at once irritable and contractile. In the 

 freshwater Hydra, however, there is the beginning of nervous 

 tissue, and in very simple expression. To the inner aspect of the 

 outer or ectodermic covering cells, which show considerable division 

 of labour, there is a loose network of nerve-cells or ganglion-cells 

 which give off delicate fibres. Some of these are connected with the 

 contractile roots of muscular-cells, and might be called motor. But 

 among the outer covering cells there are also minute sensory cells 

 with ingrowing fibres. Already there is the essence of nervous tissue. 

 In sea-anemones and some other Coclentera the division of 

 labour in nervous tissue is more distinctly defined. Superficially there 

 are sensory nerve-cells (S), which receive stimuli and also pass on 



