CHAPTER III 



SYNGAMY, EARLY DEVELOPMENT, PARTHENOGENESIS 



THE ripe microgamete or spermatozoon is a very minute motile cell, 

 highly specialized for the purpose of conveying the chromatin and centro- 

 somes from the male parent into the macrogamete. While varying 

 greatly in form in different groups of the animal kingdom, by far the 

 commonest form for it is a relatively large head containing the nucleus, 

 to which is attached a flagellum or tail. The latter is the organ by 

 means of which the movements of the microgamete in search of the 

 macrogamete are carried out. It is not attached directly to the head, 

 but through the intervention of the middle piece, which contains the 

 centrosome (in many spermatozoa). As will appear directly, the sper- 

 matozoon also presents other structural features of some importance. 



A. THE DEVELOPMENT OF THE SPERMATOZOON 



It is necessary first to consider briefly the development of the sper- 

 matozoon from the spermatid, since this matter bears upon the interpreta- 

 tion of the role of the nucleus in heredity. Before we can understand 

 either the structure or development of the spermatozoon we must have 

 some knowledge of the cytoplasmic bodies known as chondriosomes. 1 

 The nature and significance of these bodies are obscure, and are discussed 

 in Chapter VI., but they undoubtedly play an important part in the 

 structure of the spermatozoon. 



If material for cytological study be treated by appropriate methods 

 of fixing and staining, the chondriosomes are revealed as very definite, 

 though minute, bodies in the cytoplasm. Their behaviour in a particular 

 case, the insect Blatta germanica (Duesberg, 1911 a), may be taken as 

 sufficiently typical of the general course of events (Fig. 29). 



Starting with the resting spermatogonium, the chondriosomes are here 

 in the form of longer or shorter filaments distributed irregularly through- 

 out the cytoplasm. In many other species they are granular instead of 



1 See note on terminology on p. 195. 

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