30 



ZOOLOGY 



SECT. 



the corpuscles disseminated through a liquid plasma, which 

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



med 

 nea 



FIG. 21. Nerve-cells. A, multipolar ; 

 B, bipolar. 



Fm. 22. Nerve-fibres. A, medullated ; 

 B, iion-medulated ; ax, neuraxis ; 

 med, medullary sheath ; neur, 

 neurilemma. 



In a large proportion of cases such corpuscles are similar to 

 Amoebse 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 (haemoglobin) of the blood. The leucocytes are able, like 

 Amosbse, 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 plasmodium. 



The characteristic cells of the 

 reproductive tissues are the ova 

 and the spermatozoa or sperms. The 

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

 relatively large, usually spherical 

 cells, sometimes composed entirely 

 of protoplasm, but usually with an 

 addition of nutrient food-yolk. Each 

 ovum, as already mentioned, en- 



cloS6S a large nucleus (qcrmilial 

 ? \ - j / 



vesicle) and in the interior or that 



IT 7 



one or more nucleoli or germinal 



<^ s 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 ; 6, of a Turbellariaii 

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



worms ; /, of a Crustacean ; g, of a 



Salamander ; h, the commonest form 



with oval head and long flageiium. 



(From Lang's Comparative Anatomy.) 



