CONCRESCENCE IN EMBRYO OF CRYPTOBRANCHUS. 249 



brush, and keep it in any desired position on a bed of cotton; 

 but on account of the extreme delicacy of the egg, mechanical 

 manipulation is preferably to be avoided. The drawings used 

 in illustrating this paper were made with the aid of a mirror. A. 

 camera lucida was not employed, for the reason that its use 

 requires too much handling of the egg in order to get it in the 

 right position for drawing. Free-hand sketches were made as 

 accurately as possible, and it is believed that they give as faithful 

 pictures as would be the case were they made with a camera. 



III. EXPERIMENTS AND OBSERVATIONS. 



A. Convergence. In my previous work on the development of 

 Cryptobranclnis (Smith, '12), observational studies alone gave 

 evidence that a band of cells occupying an equatorial position 

 in the late blastula moves, during gastrulation, toward the vegetal 

 pole^and converges on the site of the closing blastopore to form 

 the posterior end of the embryo. By means of staining experi- 

 ments Goodale ('n) has demonstrated a similar process in the 

 eggs of Spelerpes and Amblystoma. The following experiments 

 enable us to determine more precisely the nature of this move- 

 ment and the distribution of the cells involved. 



In connection with another series of experiments some eggs 

 were stained in the early first cleavage stage by placing a mark 

 in the equatorial region opposite each end of the cleavage furrow. 

 Figs. 3 and 4 represent one of these eggs in which the first 

 cleavage furrow extended precisely at right angles to the median 

 plane of the future gastrula. In Fig. 4, representing the lower 

 hemisphere of the early gastrula, the two spots have been carried 

 ventrally and posteriorly to a position at the ends of the cres- 

 centic blastopore, which they tend to enter from below (i. e., 

 from the posterior side). In comparing these and similar figures, 

 it must be remembered that, since the egg is a spherical object 

 and the drawing a projection on a plane surface, the distance 

 through which a mark travels is in certain situations much 

 greater than appears in the drawings. 



Fig. 5 represents the lower hemisphere of an egg stained in 

 the equatorial region of the late blastula. Fig. 6, drawn two 

 days later, shows the spots on the posterior side of the egg 



