THE MIGRATION OF THE ANUS. 



95 



There is a regular change in the position of the embryo with development. 

 The blastoderm is formed at the side of the yolk. When the gastrula covers half 

 the yolk, the egg has rotated so that the gastrula covers more of the lower than 

 of the upper surface of the yolk. Still later, some hours before the closing of the 

 blastopore the latter structure lies to one side of the yolk-sphere, which always 

 occupies the upper pole of the egg ; the embryo extends from this region obliquely 

 over the yolk. After the formation of the tail the embryo is always found coiled 

 about the upper half of the yolk. The period spent in the egg lasts about a month. 

 In the laboratory some embryos hatched in about 28 days, but in the cold cave 

 streams this period would probably be several days longer. The yolk has been 

 but little affected at the time of hatching, measuring 1.8 mm., the oil-sphere about 

 1 mm. ; and since the yolk is all absorbed before the young are freed from the 

 gill membrane, probably another month is spent under the gill membrane. 



Fig. 30. (a) Internal Anatomy of A mblyopsis spdaius. 1, anus; 2, opening of oviduct; 



3, oviduct; 4, ovary, which is single; 5, liver; 6, duodenum; 7, gall 



sac; 8, pectoral fin; 9. one of pyloric ca^ca; 10, cecum; 11, stomach; 



12, spleen; 13, air bladder; 14 and 16, intestine; iq, pancreas; /., liver. 



(6) Alimentary Canal of Clwlogaster cornulus. £r., pyloric ca?ca; 5., stomach; v., vent. 



(c) Alimentary Canal of Chologaster papilliferus. 



(d) Alimentary Canal of Chologaster agassiziu 



(e) Alimentary Canal of Typhliclilhys subterraneus. 



The young, on hatching, are about 5 mm. long and lie on their sides. The 

 motion of the tail produces no effect other than to cause them to spin around 

 with the yolk for a pivot. The metamorphosis of the larva into the definitive fish 

 is completed before it leaves the gill cavity of the mother. The longest individ- 

 uals I have secured from the gill cavity measure about 10 mm. 



THE MIGRATION OF THE ANUS. 



Certain structures gain an entirely new significance in the light of the breed- 

 ing habits. These are the enlarged gill cavities with the small gills, the closely 

 applied branchiostegal membrane, and the position of the anus and sexual orifices. 



The anus in all of the species has undergone a curious translocation. The 

 primary cause of the transposition probably lies in the ovary and oviduct, and not 



