274 FINE-STRUCTURE OF PROTOPLASM II 



inferred from either the indirect methods, or from direct electron 

 microscopy, alone, but that the two modes of enquiry should be 

 complementary and the results obtained with the one should be 

 scrutinized in the light of the data produced by the other. 



§ 5 . Gametes 



Gametes are very highly differentiated cells with the faculty of 

 transmitting to the zygote the capacity of developing all the pro- 

 spective properties of the future organism. For that reason, their sub- 

 microscopic structure is of particular interest. The results attained in 

 this direction are still rather scanty; but there are already some 

 interesting electron microscopic investigations on the fine-structure of 

 gametes which are reviewed below. 



a. Spermatozoa 



The tails of certain spermatozoa are positively birefringent, whereas 

 their heads are negative (Schmidt, 1937a). The inference is that the 

 anisotropy of the tail is due to protein fibrils, that of the head to the 

 inclusion of orientated nucleic acid (Fig. 125a, p. 228). 



The head of the sperm being too thick for the transmission of 

 electrons in the electron microscope, only details of its surface can be 

 explored; but the thinner tail offers excellent conditions for such an 

 investigation, and the submicroscopic structure of this part of 

 spermatozoa is now thoroughly known. 



Using the microscopic information available, Bretschneider 

 (1949b) has drawn the diagram of Fig. 137 as a result of his electron 

 microscopic investigations. A strong nuclear membrane of protein 

 fibrils causes the characteristic shape of the head, which contains the 

 chromosomes. It is enveloped by a thin layer of cortical cytoplasm. 

 The apex is covered by the so-called head cap consisting of a highly 

 hydrated gel that plays an important role in fertilization (Bret- 

 schneider, 1950b). Its distal end is marked by a sharp line in the 

 cortical plasm. The basal part of the head is covered by a very thin 

 sheath, the head tunica. There is a collar formed by a ring-shaped 

 membrane around the base of the head, where the tail is fastened. 



The tail consists of 9 microfibrils into which the axon of the 



