150 BULLETIN OF THE UNITED STATES FISH COMAUSSION. 



sented id Fig. 10, begins to exhibit some likeness to an animal organ- 

 ism, but as yet the species would not be recognizable were it not known 

 from what parent form the egg had been obtained, though it could 

 undoubtedly be referred to the class Pisces, subclass Tclcostei. A care- 

 ful study, however, of a number of forms will enable the student to 

 distinguish them apart at a very early period of development. The 

 number of somites or muscle plates increases from before backwards; 

 they first appear some distance behind the rudiments of the ear, au 

 Fig. 8, and by the regular successive segmentation of the upper meso- 

 blastic plates of cells on either side of the medullary canal increase in 

 number backwards toward the tail, and by the time the caudal swell- 

 ing is developed there are about twenty muscular somites or segments 

 of the body. As the tail grows and is extended backwards the seg- 

 mentation of the muscular stratum of the mesoblast continues in the 

 same way from before backwards, but does not for a considerable time 

 involve all of the structure destined to become the lateral muscular 

 masses of the young fish; the i^ortion at the end of the tail remaining 

 uusegmeuted for some time after hatching. The muscular segments, 

 somites, or protovertebriE, as they have been variously named, originate 

 from the two tracts of mesoblastic tissue on either side of the medullary 

 canal, and are really the rudiments of the muscular masses, the edible 

 flesh portions found on either side of the backbone of adult fishes. They 

 appear at first in the embryo as quadrate masses lying on either side of 

 the medullary canal, and in embryo sharks, according to Balfour, and in 

 the lampreys, according to Scott, at first are said to be hollow; in our 

 studies we have not succeeded in demonstrating this peculiarity in 

 teleosteau embryos. They are nearly or quite solid in the latter. 



Immediately after the tail swelling has been formed, the caudal rudi- 

 ment forms a blunt rounded point, which, as it is prolonged backwards, 

 develops a continuous median dorsal and ventral ridge or fold of epi- 

 blast, as shown in Fig. 13, and which becomes the natatory fold nf of 

 Fig. 13, from which all the unpaired fins are developed. Almost as soon 

 as the tail begins to grow out a strand of hypoblastic cells, v, Fig. 12, 

 is seen on the lower side of it, lying between the epiblastic layers of the 

 natatory fold, and extending to the edge of the latter; this strand of cells 

 appears to have been i^robably continuous with the medullary canal or 

 cord on the dorsal side of the embryo before the tail swelling grew out, 

 so as to break and obliterate its connection with the former. This strand 

 of cells, which is seen to be apparently tubular, is the anal extremity of 

 the gut, and seems to be closed posteriorly. It is found to extend for- 

 wards, as development proceeds, as a fiattened tube, lying Just below 

 the notochord or cartilaginous axis of the embryo as shown at /, Figs. 12 

 and 13. The intestine was probably continuous with the medullary 

 canal posteriorly, from the hinder extremity of which it has possibly 

 been invaginated from above, in which case the ffastnda stage of devel- 

 opment of the teleosteau fish embryo would be perfectly homologous 



