360 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



SPERMATOGENESIS AND THE SPERMATOZOA. 



Prof. George W. Field gives the following account of the spermatogenesis and 

 of the spermatozoa of Antedon mediterranea. For descriptive purposes he divided 

 the mature spermatozoan into the following parts : 



(Nucleus 1 _ , 

 _ ^ V Head proper. 

 Centrosome.j 

 Mitosome 1 -,.,,, . 

 „ ,, . ) Middle piece. 



Cell membrane_j 



Tall. 



The nucleus of the spermatozoa of Antedon mediterranea measures in longi- 

 tudinal diameter 3.3 [a and in transverse diameter 1.3 p.; the centrosome is 0.66 \i. in 

 diameter, and the mitosome is 1.3 [a. 



The nuclei of the spermatogones in the outer zone, as seen in a section of the 

 testis, usually alone of all the cells show nucleoli. 



Considering the amount of cytoplasm the nucleus is relatively large. It very 

 soon begins the process by which it will ultimately give rise to the spermatozoa. 

 It is divided by mitosis and forms the nuclei of two spermatocytes. The number 

 of chromosomes into which the nuclein of the spermatogone collects seems to be 

 from 28 to 36; in the spermatocyte, from 16 to 18. The attempt to count with 

 exactness such small and numerous bodies so closely crowded together is well nigh 

 fruitless. 



The dividing nucleus is seen plainly to be made up of several substances, as for 

 example, nuclein, the caryoplasma, and another substance which has the appearance 

 of minute granules; these evidently form the mitotic spindle. A centrosome is 

 also present. 



The nucleus of each spermatocyte has the same constituent parts as that of 

 the spermatogone. It divides by mitosis and forms the nuclei of two spermatids. 

 The nucleus of each spermatid contains eight or nine chromosomes and caryo- 

 plasma. Within the nucleus there seems to be no sign of the granules which 

 formed the nuclear sjjindle, but these granules and the centrosome are now very 

 distinctly seen to he in the cytoplasm. It should be noted that each of these 

 mitoses are reducing divisions. 



With the spermatid begin those changes in the shape and constitution of the 

 nucleus which are connected with the specialized form of the spermatozoon. The 

 nucleus gradually changes from spherical to conical. There seems to be a change 

 in the constitution of the nucleus. The chromosomes (usually 9, sometimes 8) 

 can be demonstrated in the spermatids and in certain of the spermatozoa, probably 

 the immature ones. But in others the nucleus remains homogeneous under the 

 same reagents and conditions which demonstrate the chromosomes in the others. 

 These spermatozoa with the homogeneous nuclei are the most active, and most 

 frequently penetrate into the ovum. In sections of fertilized eggs the nucleus of 

 the spermatozoon when in the outer zone of the cytoplasm of the ovum is small, 

 dense and homogeneous; on the other hand, the nucleus which has traveled for 

 some distance toward the female pronucleus is considerably larger and shows 



