28 BERTRAM G. SMITH. 



III. The Embryo. 



An abundance of fertilized eggs was collected, but on account 

 of the lack of proper facilities great difficulty was experienced in 

 keeping the material alive long enough to secure a complete 

 series. By far the greater part of the eggs perished from varia- 

 tions in temperature, insufficient aeration, or the attacks of 

 water-mold, and it was only by constant search for new nests 

 that the later stages were obtained. 



Segmentation. — The segmentation stages have already been 

 described (Smith, '06 2 ), and since I expect to make a detailed 

 study of the embryology the subject of a special paper, only a 

 few brief notes on the development of the embryo need be given 

 here. 



In artificially fertilized eggs the first cleavage furrow appears 

 about 1 8 hours after fertilization. Segmentation proceeds with in- 

 creasing rapidity, varying with the temperature, and in a few days 

 a blastula is formed having a shallow segmentation cavity with 

 a very thin, almost transparent roof. So far as external changes 

 visible to the unaided eye are concerned, development during 

 the latter part of this process is comparatively slow. It is also a 

 critical period in the development of the egg ; more embryos die 

 at this time if exposed to slightly unfavorable conditions than at 

 any other. 



Gastrulation commences in from six to ten days, according to 

 temperature, after fertilization. The blastopore is formed much 

 as in the frog's egg. The process is quite rapid after the appear- 

 ance of the short deep horizontal groove just below the equator; 

 this groove quickly elongates, becomes crescent shaped, and the 

 horns close to forma complete circle in some eggs, while in 

 others of the same lot subjected to the same conditions, the proc- 

 ess of gastrulation has just begun. The yolk plug rapidly 

 diminishes in size. 



Origin of the Nervous System. — The broad but shallow neural 

 groove appears at about the time the circle of the beginning blas- 

 topore is nearly complete — a day or two after gastrulation com- 

 mences. A few hours later the neural folds appear as a sharply- 

 defined horse-shoe shaped ridge about the upper end of the 

 neural groove ; the arms extend downward and closer together 



