16 bulletin: museum of comparative zoology. 



at about the same time in the life of the parasite as was the epithe- 

 lium of the appendix or of the appendicular vesicle as observed by 

 Pratt. 



Male Sexual Organs. 



The testes (Fig. 1, te.) are spheroidal, and lie anterior to the middle of 

 the body and a short distance posterior to the ventral sucker. They are 

 obliquely placed, the right one being anterior to the left, and each lying 

 only a little to one side of the median plane of the body. The trans- 

 verse diameter of each of the testes, which as a rule is slightly shorter 

 than its longitudinal diameter, is about one-third as great as the width 

 of the body, their average measurements in five specimens being 0.147 

 mm. by 0.129 ram. The wall of the testis is an apparently homogeneous 

 membrane, in which I have been unable to find nuclei ; its cavity is 

 ordinarily nearly filled with primitive germ cells, groups of spermatozoa, 

 and sperm-morulae, or groups of cells in various intermediate stages of 

 development. The primitive germ cells do not form a continuous 

 peripheral layer, as described by Wright and Macallum ('87) in Sphyra- 

 nura osleri, but appear as numerous syncytial masses lying in the 

 peripheral portion of the testis, as described by Looss ('85) in Distomum 

 palliatum, and by Schwarze ('85) in Distomum endolobum. The 

 nuclei in these syncytia may be arranged in a single layer, but com- 

 monly they form a more or less irregular group projecting some dis- 

 tance toward the centre of the organ. No cell boundaries are to be 

 distinguished in these masses. The nuclei have a diameter of about 

 4 fx. Cells are apparently detached from these syncytia and undergo 

 development into spermatozoa in the more central portion of the testis. 

 Each of these proliferated cells divides, forming successively groups of 

 two, four, eight, sixteen, and thirty-two, the cells and their nuclei becom- 

 ing smaller as their number increases (Plate 2, Figs. 15-17). I believe 

 that, as a rule, all the cells of a group divide synchronously, since all 

 the cells of any group are very nearly equal in size and their nuclei are 

 in the same condition. Kerbert ('81) states that the number of nuclei 

 found in the sperm-morula of Distomum westermanni may reach six. 

 Schwarze ('85) found as many as sixteen in Distomum endolobum, while 

 Wright and Macallum gave the number found in Sphyranura as being 

 over forty. Schwarze could not determine whether each nucleus of the 

 sperm-morula was surrounded by a mass of its own cytoplasm or not; 

 but Wright and Macallum found cells radiating from the centre of the 

 mass, the nuclei being peripherally situated. 



