3GG THE TESTIS. [BOOK iv. 



dinal, with often a third innermost longitudinal layer of muscular 

 fibres making their appearance ; the epithelium here consists of 

 several layers of cells, the uppermost of which are columnar, but 

 the cilia disappear. 



940. The seminal tubules form the true secreting portion 

 of the testis ; the vasa recta mark the junction of the germinal 

 with the Wolffian elements, and from thence onward all the rest 

 of the organ is conducting in nature ; it is in the seminal tubules 

 that the spermatozoa make their appearance, and we may now 

 turn our attention to these. 



If a section of ripe testis be examined under a moderately 

 high power of the microscope, no great difficulty will be found 

 in observing the following features. Each tubule is defined by 

 a "basement membrane, generally consisting in man of more than 

 one lamina, imbedded in which, as in the case of so many other 

 basement membranes, the constituent nuclei may be seen. Upon 

 the basement membrane rest several layers of epithelium cells ; 

 the cells are obviously not all alike, and some are obviously under- 

 going changes. The centre of the lumen of the wide tube (200 ^ 

 in diameter) is occupied by bundles of spermatozoa ; and it may be 

 seen that each spermatozoon consists of an enlarged head, and a nar- 

 row tapering tail, and further that the bundles of spermatozoa are so 

 arranged that the heads seem plunged among the epithelium cells 

 while the tails converge towards the centre of the lumen. There 

 can be no doubt that the spermatozoa are formed in some way 

 out of the epithelium cells lining the tubule ; but the exact way 

 in which they are formed has been, and still is, the subject of 

 much discussion. We must be content with briefly describing 

 what appears to be the best supported view. 



A spermatozoon consists in the first place of a homogeneous 

 (or for the most part homogeneous) highly refractive head, which 

 in man is a flattened, somewhat curved oval (about 5/A long), but 

 which differs in shape and size in different animals. From the 

 hind pole of this oval head there proceeds a delicate filament (50/z. 

 in length in man) tapering to a point ; this is the tail which, ob- 

 viously of a different nature from the head, possesses the power of 

 spontaneous movement, and in many respects resembles a cilium. 

 The portion immediately succeeding the head is thicker than, and 

 in other ways differs from the rest of the tail ; it is often dis- 

 tinguished as the middle piece, body, or neck. In the newt the 

 tail for the greater part of its length consists of a delicate mem- 

 brane with a thickened edge wound spirally, like a miniature spiral 

 fin, round a central thread ; and something similar is seen in some 

 mammalia but not in man. We may at once say that there is 

 every reason to regard the head as a specialized nucleus, or part of 

 a nucleus, and the tail as a remnant of a cell-body. 



941. In a seminal tubule, a cell of the outer layer next or 

 near to the basement membrane undergoes the changes in the 



