MALE REPRODUCTIVE CELLS 427 



into a bi-lobed chromosome so that both lobes must be represented by 

 AB. The first longitudinal division, if the separation really takes place 

 along the line of original fusion, yields two chromosomes, A and B, and 

 there has been a true reduction, and the case is one of pre-reduction. 

 The second longitudinal division simply divides chromosomes A and B 

 into similar chromosomes of half the original size, and the division is 

 an equation division. In many cases one of the maturation divisions is 

 transverse, but, however the divisions take place, the important fact as 

 to whether the divisions are qualitative or quantitative merely depends 

 upon the manner of union of the chromosomes in synapsis. These two 

 important divisions have resulted in four spermatids. 



The great temporary differentiation, which the male reproductive 

 cell undergoes to adapt itself for motion, is now developed, and trans- 

 forms it from a spermatid into the final form, the spermatozoon ( Fig. 381). 



The spermatozoon is a cell which has a variety of forms in different 

 animals and these variations of structure can be best studied and under- 

 stood when it is remembered that they have for a common object the 

 transportation of the important nucleus of this cell to the ovum, its 

 entrance into this ovum, and the final apposition of its chromosomes 

 with those of the ovum. 



There are two ways in which this transportation is accomplished : 

 by the amoeboid movements of an undifferentiated cytoplasm as in the 

 spermatozoa of some Crustacea (Fig. 381, H); and by the development 

 of one or more flagella or permanent cytoplasmic processes which pro- 

 pel the cell by swimming movements instead of by crawling, as must be 

 the case in amoeboid cells (Fig. 381, all forms except H). By far the 

 larger number of spermatozoa are of the swimming kind, and most of 

 these are propelled by a single strong flagellum. In the analogous male 

 reproductive cells of plants there are oftener two flagella. We shall 

 first concern ourselves with the structure of the more typical flagellate 

 spermatozoon, as shown in the diagram to the right in Figure 381. 



In this form the nucleus appears as a compressed oval body which 

 is placed in an anterior cytoplasmic enlargement, the head. The nucleus 

 is composed of the reduced number of individual chromosomes, of 

 course, but they are indistinguishable at this time, forming a solid chro- 

 matic content of the nucleus. The head and its contained nucleus is 

 not always oval, but may be much elongate, corkscrew-shaped (381, D), 

 or formed like a horseshoe (381, C). The head terminates in a cap- 

 like structure frequently sharp, occasionally blunt, called the acrosome. 



The heaviest mass of the cytoplasm lies behind the head, and is known 

 as the middle-piece. It is the cell body of the spermatozoon and contains 

 the future centrosome, if this body is carried over structurally from 

 generation to generation. The middle-piece is developed from the 



