BULLETIN OF THE UNITED STATES FISH COMMISSION. 299 



boliind with the iieurula or spinal canal, as in the Amphibian and Mar- 

 sii>obranch, and there no evidence to show that the point where the rim 

 of the blastoderm closes is comparable to a blastopore, or to the auns of 

 Rusconi. 



7. The gastrula of the teleost is extremely modified on account of the 

 extreme flattening and epibolic mode of growth of the blastodeim over 

 the yelk, but tlie type of development is, in reality, similar to that where 

 there is a neurenteric canal developed as in embryo sharks, lampreys, 

 and frogs, since the vent is always broken through long before the 

 mouth, and there is a strand of cells representing the neurenteric canal. 



8. The blastoderm of the teleost may be regarded as a very depressed 

 concavo-convex hollow sack resting on the yelk, the hollow space beneath 

 it representing the persistent cleavage cavity. One side of the blasto- 

 dermic disk or sack is filled with mesoblast cells, from which the somato- 

 pleure and splanchuoplenre are derived, where the embryo is formed; 

 the intestinal lumen is, at first, a narrow transverse split in the hypo- 

 blast which extends forward, eventually prolonging the enteric cavity 

 beneath the head. 



9. The uppermost or epiblastic layer of the blastoderm, several cells 

 deep, roofs over the cleavage cfivity, the hypoblast forms its floor, the 

 rim of the blastoderm contains mesoblastic cells, which, as the germinal 

 membranes close over the yelk form the caudal-plate which is continuous 

 on either side with the medullary or muscle-plates at the sides of the body 

 of the embryo. The caudal-plate eventually enters into the formation 

 of the tail and caudal muscular mesoblastic somites, its hyi^oblast into 

 the formation of the anal end of the intestine. 



10. The cause of the at first flattened lumen of the intestine is probably 

 to be sought in the very depressed and modified type of blastoderm of the 

 teleostean, which diflers widely' from that of all other vertebrates. The 

 lumen of the intestine gradually becomes round. 



The embryo develops at the edge of the blastoderm in Teleosts, 

 Elasmobranchs, and Ganoids, but only a small portion of the blasto- 

 dermic rim appears to be appropriated to form the embryo in the Elas- 

 mobranch. This eccentric develojuuent of the embryo is in strange 

 contrast with that of the Amphibian and Lamjjrey, and not less so 

 when compared with the mode of development of reptiles, birds, and 

 mammals where the embryo develops in the center of the blastoderm, 

 and where the yelk, when present, ai3i)ears to be merely nutritive and 

 accessory, as in the teleostean egg. Only in the case of Zoarces is there 

 an approach toward the formation of an umbilical stalk, according to 

 Ratlike, but even there it is not developed until some time after the 

 blastoderm has closed over the yelk. 



