58 HUBERT LYMAN CLARK ON 



4. FERTILIZATION AND SEGMENTATION OF THE EGG. 



Like all other known Synaptas, S. vivipara is hermaphroditic. Ripe spermatozoa and 

 ova are found in the same genital organ and even in the same branch, and to judge both 

 from living material and preserved specimens, even at the same time; but this latter 

 point could not be proven since it is impossible to determine simply by observation, when 

 the egg is mature. The branches of the genital gland are nile-green in color in the living 

 animal, while the fully grown ova are brownish yellow, and the spermatozoa, when in 

 any quantity, appear to be white. In shape, the latter are like those figured by Jourdan 

 ('83) for Holothuria tubulosa. The mature egg (Fig. 1) is about '20(V in diameter and 

 before fertilization is not provided with any membrane. It is full of yolk material but 

 comparatively transparent, so that the internal changes could be watched up to a late 

 stage of development. Artificial fertilization proved unsuccessful, although attempted 

 several times. Whether self-fertilization takes place or not could not be positively 

 decided, but that it is at least very improbable seems clear from the structure of the 

 genital organs and the probable manner of egg-laying. The genital gland of the adult 

 Synapta lies just above the oesophagus, with one or two branches on each side of the 

 dorsal mesentery and with the genital duct lying in that mesentery and opening to the 

 exterior close to the base of one of the mid-dorsal tentacles. A cross-section of one of 

 the branches (Fig. 40) shows that it consists of an external covering, the continuation of 

 the epithelium, a very scanty connective-tissue layer consisting only of a few scattered 

 mesenchyme cells, and a much folded germinal epithelium surrounding the lumen of the 

 gland. Of the layer of circular muscle fibers which was found in the genital gland 

 of S. digitata and S. inhaerans by Quatrefages ('42), Baur ('64), and Hamnnn ('84), I 

 have found no trace in S. vivipara. The branches of the gland contract vigorously after 

 being cut from the body, but so far as I could see, these contractions were always longi- 

 tudinal. The germinal epithelium is more or less plainly made up of two or more layers 

 of cells of which the external are the larger. As will be seen from figures 39 and 40, 

 from this external layer of cells the ova arise and so come to lie between the germinal 

 epithelium and the epithelium of the body-cavity, and do not pass into the lumen of the 

 gland at all. This arrangement is quite the reverse of what Cuenot ('91) has shown to 

 exist in S. inhaerans, and gives us a clue as to how the eggs get into the body-cavity of 

 the mother. When the eggs are mature, they press so closely against the external 

 epithelium of the gland that it bulges out sufficiently to be seen with the naked eye, in 

 the living gland. Sections show that the epithelium over such ova is so stretched as to 

 be thinner than elsewhere (Fig. 39), and probably the eggs enter the body-cavity simply 



