266 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



its organs. The posterior end of the embryo, which toward the comple- 

 tion of gastrulation was broader than the anterior end, becomes narrower 

 and narrower, and ultimately forms the tail, which is curved ventrad 

 around the trunk of the embryo within the egg membranes. 



Before the closure of the blastopore the chorda is a plate of cells lying 

 in the dorsal wall of the archentei'on, anterior and lateral to the blasto- 

 pore. The portions lateral to the blastopore meet in the median plane 

 when the blastopore closes. The chorda fundament then elongates, 

 owing to a shoving together of its cells from each side, "like a pack of 

 cards in shuffling " (Van Beneden et Julin), until they form, instead of 

 a plate, a single median row of disk-shaped cells arranged one behind 

 another like a row of coins and reaching backward underneath the nerve 

 cord to the extreme posterior end of the embryo. Anteriorly it termi- 

 nates not far from the middle of the trunk region. 



The muscle cells, which originally lay on each side of and behind the 

 blastopore, extend themselves a single cell deep in two bands, one on 

 each side of the chorda throughout its entire length. 



The mesenchyme cells originally formed the lateral portions of the 

 chorda-mesenchyme ring. As the blastopore gradually closed, they were 

 thrust down to a deeper level than the muscle cells, and forward. Ulti- 

 mately they come to lie wholly in the trunk region, chiefly in its pos- 

 terior portion, in two pretty compact lateral masses of small deeply 

 stained cells, two or more layers deep. At a still later period, these 

 lateral masses are resolved into migratory cells, i. e. blood corpuscles, 

 mantle cells, etc. 



Before the closure of the blastopore the endoderm forms the entire 

 lining of the archenteron in its most anterior portion, where its lumen is 

 almost obliterated. Farther hack the chorda forms the dorsal wall of 

 the archenteron, the mesenchyme cells forming its sides, the floor only 

 being occupied by the endoderm cells. In the region where the blastopore 

 closes, the endoderm cells occur only as a double row ventrally situated 

 along the median line. 



This double row is extended back in the larva underneath the chorda 

 throughout almost the entire length of the tail, forming a " subchordal 

 endoderm strand," which is ultimately resolved into wandering cells, 

 or perhaps utilized as food material by the mesenchyme cells of the 

 trunk region. At the posterior end of this caudal endoderm strand lie 

 the small mesoderm cells which Van Beneden et Julin mistakenly 

 included in the nervous fundament. These cells are to be regarded as 

 the most posterior constituents of the original chorda-mesenchyme ring. 



