6 TEXT-BOOK OF EMBRYOLOGY. 



polylecithal (certain fishes, reptiles, birds) . It has been stated earlier in the 

 chapter that in case the yolk is accumulated in greater quantity toward one 

 pole the ovum is telolecithal, while in case of nearly uniform distribution it is 

 homolecithal. The yolk has a slightly higher specific gravity than the 

 cytoplasm, in consequence of which the animal pole of the egg turns. upward, 

 except in most of the teleost ova where the yolk is composed of oil droplets 

 that are lighter than the cytoplasm. In many insect eggs the yolk is cen- 

 trally placed and the cytoplasm forms an outer layer; these are known as 

 centrolecithal ova. 



THE SPERMATOZOON. 



Compared with the ovum the spermatozoon is an exceedingly small cell 

 bearing little resemblance to the ordinary or typical cell. It is so small 

 in most animals that the ovum of the same species exceeds it in bulk several 

 hundred thousand times. Its peculiar shape and structure are correlated 

 with its high degree of motility, the cytoplasm being drawn out into a long 

 slender tail or flagellum which in the living cell is lashed about and thus 

 drives the whole cell along. All spermatozoa of vertebrates are of the 

 flagellate type, the human spermatozoon serving as an example. 



With the usual preparation the human spermatozoon shows a head, a 

 middlepiece or body, and a tail, measuring in total length from 50 to 60 micra. 

 On side view the head is nearly oval, usually a little narrower at the front 

 end; on edge it appears pear-shaped. The nucleus is situated in the head, 

 nearer the attachment of the body, and a thin layer of cytoplasm, the galea 

 capitis, surrounds the nucleus and is continued forward as the acrosome. The 

 head is about 4.5 micra in length, 2 to 3 in width and i to 2 in thickness, being 

 much smaller than a red blood corpuscle. The body is attached to the 

 broader end of the head and is cylindrical, measuring about 6 micra in length. 

 Sometimes a narrower portion, the neck, is visible at the point of attachment. 

 Without sharp demarkation the body continues into the slender tail which 

 runs to a point and measures from 40 to 50 micra in length. 



Special preparations of spermatozoa reveal other details of structure 

 (Fig. 4). The body contains a delicately fibrillated cord, the axial thread, 

 which is continued throughout the tail, narrowing to a point at its terminus. 

 Surrounding the axial thread is a capsule of cytoplasm which, however, does 

 not extend to the tip of the tail, thus leaving the axial thread naked for a 

 short distance. In the body the cytoplasm contains a spiral fiber, perhaps 

 of a mitochondrial nature, winding round the axial filament; other mitochon- 

 dria also are present. The body contains the centrosome which takes the 

 form of a double structure; one part, the anterior end knob, is attached to 

 the posterior surface of the head close to the nucleus, the other part, the 



