DEVELOPMENT OF AMPHIUEA SQUAMATA. 143 



the lumen appears to me to come in consequence of the 

 increasing periphery of the organ. It is possible, however, that 

 some of the internal primitive germ-cells divide completely to 

 form spermatozoa, and leave no part to regenerate the mother- 

 cell. Young testes are often quite a solid mass of sperma- 

 tozoa and sperm mother- cells, and no trace of degenerate cells 

 is ever visible. 



The genital bursse originate late as invaginations of ectoderm 

 (Russo, 16). There is, however, a very curious point about 

 their development hitherto unnoticed. The first sign of their 

 appearance is an increase in size and number of the peritoneal 

 cells (fig. 19). Afterwards the ectodermal invagination is 

 apparent (fig. 20). This first rudiment of the bursa is often 

 very like a portion of the genital rachis ; and as it is found at a 

 stage when this is imperfectly or not at all formed, it is apt to 

 lead to confusion. The best methods of distinction are of 

 course the absence of a sinus, and its non-persistence on suc- 

 ceeding sections. When looking for specimens with a rudi- 

 mentary rachis, one must carefully distinguish this rudiment 

 of the bursa. 



Literature consulted. 



I. Ovoid Gland. — " Hsemal and Perihajmal Systems." 

 In discussing this subject I think it neither necessary nor 

 profitable to go back beyond the time of Ludwig, as before his 

 time the ovoid gland was either not recognised or confounded 

 with a Polian vesicle ; whilst any space that happened to be 

 seen in cross-section was called indiscriminately a blood- 

 vessel. Ludwig (10, 11) not only distinguished the ovoid 

 gland and homologised it with the organ occupying a similar 

 position in other Echiuoderms, but discovered the ampulla 

 and distinguished it from the axial sinus ; found the aboral 

 sinus with its contained blood-vessel and rachis, and the 

 relation of these to the genital organs. He also described an 

 oral blood-ring and radial blood-vessels. The ovoid gland he 

 regarded as a plexus of vessels uniting the oral and aboral 

 blood-rings. 



