30 



ZOOLOGY 



SECT. 



— the cm-puscles — disseminated througii a liquid plasma, which 

 takes the place of the ground-substance of the connective tissues. 



3a , Tieur 



Fig. 21. — Nerve-cells. A, multipolar ; 

 JJ, bipolar. 



Fio. 22. — Nerve-fibres. A, medullated; 

 B, lion-niedulated ; ax, iieuraxis ; 

 ?>ie(Z, medullary sheath ; iiewr, 

 neurilemma. 



In a large proportion of cases such corpuscles are similar to 

 Amoebae in their form and movements (amoeboid corpuscles, leuco- 

 cytes). In the blood of Vertebrates leucocytes occur along with 

 coloured corpuscles of definite shape containing the red-colouring 

 matter (hcemoglohin) of the blood. The leucocytes are able, like 

 Amoebae, to ingest solid particles, and under certain conditions a 



number of them may unite to- 

 gether to form a single mass of 

 protoplasm with many nuclei, 

 termed a plasviodmm. 



The characteristic cells of the 

 reproductive tissues are the ova 

 and the spermcttozoa or sperms. The 

 ova (Fig. 6), when fully formed, are 

 relatively large, usually spherical 

 cells, sometimes composed entirely 

 of protoplasm, but usually Avith an 

 addition of nutrient food-yolk. Each 

 ovum, as already mentioned, en- 

 closes a large nucleus {gcrmiiud 

 vesicle) and in the interior of that 

 one or more nucleoli or germinal 

 spots. The sperms (Fig. 23) are 

 extremely minute bodies, nearly 

 always motile, usually slender and whip-like, tapering towards 

 one extremity, and commonly with a rounded head at the other. 



Fig. 23. — Various forms of spermatozoa. 

 a, of a Mammal ; &, of a Turbellarian 

 worm ; c, and d, and e, of Nematode 

 worms ; /, of a Crustacean ; j/, of a 

 Salamander ; h, the commonest form 

 ■with oval head and long fiagellum. 

 (From Lang's Comparative Anatomy.) 



