ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 



the red colouring matter of the blood, hcemoglobin, which plays so 

 important a part in respiration. They arise in all probability from 

 the colourless corpuscles which are always far less numerous in 

 normal blood. The colourless corpuscles are genuine cells of variable 

 form, and have the power of amreboid motion (migration into tissues, 

 regeneration of tissues, etc.) ; they come from the lymphatic glands, in 

 which they arise as lymph corpuscles, and eventually pass with 

 the lymph stream into the blood. The ova and spermospores, after 



PIG. 20. Spermatozoa, a, cf Medusa, b, of a Nematode. c, of a Crab, d, of Torpedo, 

 e, of Salamander (with undulating membrane). /, of Frog, g, of a Monkey (Cerco- 

 pithecus). 



they have separated from the epithelial layer in the wall of the ovary 

 and testis, as well as the spermatozoa produced from the spermospores, 

 respectively belong to the category of free cells. The form and size 

 of the spermatozoa present great variations. They always consist of 

 a modified cell, frequently of a very small cell with a long nagellum, 

 nucleus, and remains of protoplasm. In many cases the head is 

 elongated into a fibre-like structure, or is twisted like a corkscrew 

 (Birds, Selachians). Sometimes a distinct head is absent, and the 

 spermatozoon is thread-like (Insects). In the Nematodes the sperm- 



3 



