190 ESSENTIALS OF ZOOLOGY 



are shed into the peritoneal cavity. They enter the common 

 anterior end of the oviducts or Miillerian ducts, the paired 

 openings described above having merged to form a single 

 opening. The oviducts are well developed and have separated 

 completely from the mesonephric duct. They open separately 

 into the cloaca on each side. The kidney is placed as in the 

 male, but the ureters on each side open into a urinary sinus 

 and the latter into the cloaca by a urinary papilla. The 

 remains of the primitive mesonephric duct may be traced 

 forwards as two fine tubes from the urinary sinus. Moreover, 

 in the male a vestigial remnant of the anterior end of the 

 oviduct occurs on the ventral aspect of the oesophagus. 



Development. The eggs are very large in all Elasmo- 

 branch fishes. The cytoplasm is distended with yolk to a 

 degree similar to that of birds, and the nucleus is left at the 

 animal pole as a very small element in a very large cell. In 

 the case of the ovum of Acanthias taken from the ovary the 

 nucleus was calculated to be only ^^Q 000 th of the bulk of 

 the ovum. The egg is protected by two membranes. The outer 

 appears to be derived from the follicular cells and would there- 

 fore be called a chorion. The inner is secreted by the cyto- 

 plasm of the egg and is therefore a vitelline membrane. When 

 the egg is ripe it is burst from the follicle into the peritoneal 

 cavity and it is carried forwards to the internal opening of 

 the oviduct. In the upper part of the oviduct the ripening 

 process is finished and the two polar bodies are placed outside 

 the cytoplasm at the animal pole. Spermatozoa gain entrance, 

 and one of the male pronuclei fuses with the female pro- 

 nucleus. The rest are relegated to the side, ultimately 

 disappearing. The spermatozoa are long and flagellate, with 

 elongated heads. 



The glands in the upper part of the oviduct secrete albumen 

 which is coated over the egg, and the egg is then conducted 

 through the oviducal gland, where a horny case is secreted, 

 and this completes the envelopes. 



As in the case of the bird, therefore, the egg is protected 

 by envelopes developed in the ovary, and secondarily by 

 albuminous and shell coverings by the glands of the oviduct. 

 The albumen and the horny case are little developed or absent 



