1889.] On the Early Development of Lepidosteus osseus. 113 



The lateral or branchial sense organs are also products of the 

 formative epiblast. 



The cranial and spinal ganglia are formed as recently described by 

 me in sharks and birds. 



In the month involution also the formative epiblast is alone con- 

 cerned, the covering layer passes over from the anterior end of the 

 head to the yolk sac. The month breaks through about the fourth 

 day. The anus arises as a solid ingrowth of the formative epiblast in 

 the place where the blastopore once existed. 



The Larval Suckers. — These organs also are products of the inner 

 or formative epiblast. They are developed very early, and commence 

 to be differentiated about the third day of egg development. They 

 are formed as a number of closed spherical sacs, one part of the wall 

 being thin, and this part is ruptured when they break through the 

 skin a day or two before hatching. One of these sacs is median, and 

 arises almost within the mouth involution. It could almost be 

 mistaken for the developing hypophysis, were it not for its subse- 

 quent fate and for the fact that it is directed forwards. When it 

 breaks through the skin it forms a flat glandular plate on the roof of 

 the mouth. From its position it can hardly be of much use as a sucker. 



The functional suckers are composed of two sorts of cells : 

 (1) Long glandular cells with hyaline slightly granular contents 

 and nucleus lying near the inner end of the cell. Between these (2) 

 supporting cells with nucleus in the middle of the cell. 



Each glandular sucker has the structure of and is probably serially 

 homologous with the suckers of larval Anurous Amphibia. During 

 the period at which they lie beneath the covering epiblast, and 

 for a short time after, the arrangement of the suckers is distinctly 

 bilateral. The median one appears to be unpaired at all stages. They 

 form a complete circlet on the disk, with additional suckers within 

 the circle. On the whole, the organ may be said to be composed 

 of two fused circlets of suckers. Balfour and Parker noticed the 

 numerous vascular channels in the mesoblast just inside the suckers. 



The young Lepidosteus I possessed did not make use of the suckers 

 for several days after hatching.* The suckers commence to degene- 

 rate when the young fishes are about three weeks old. 



The pad at the end of the adult snout occupies the position of, but 

 cannot be considered as arising from, the larval suckers. The latter 

 are transient organs. 



Hypohlastic Organs. — The origin of the notochord is not yet clear 

 to me, for it arises during an epoch in which the processes of develop- 

 ment are difficult of interpretation on account of the yolk. 



There is at first a solid oesophagus, as Balfour and Parker state, 

 and the posterior gill clefts arise as solid evaginations from it. The 



* The first batch of eggs hatched on the ninth day, the second on the seventh. 

 VOL. XLVI. J 



