414 



Fig. 1. 2.5 mm embryo. 

 Through anterior trunk region. 

 X 400. /( hypoehordal rudiment. 

 ch notochord. en hypoblast. 



ope first — the notochord is cut off and is perched upon a shallow 

 ridge (Fig. 1), where in a 2 mm embryo it lies in as shallow a groove, 

 the change being a manifestation of the upraising of chorda-hypoblast. 

 Lying partly within the ridge is a group of three cells only the nuclei 



of which are shown. They represent the 

 hypoehordal rudiment. They are not, on 

 the average, so elongated as the hypo- 

 blast cells, yet there has not been any 

 very decided change in shape. As we 

 pass forward through the sections from 

 that described, we find that the ridge 

 gradually disappears; in other words, 

 the hypoehordal cells come nearer to 

 their more primitive position against the 

 lumen of the archenteron, for the gut is 

 still archenteric in nature at the anterior 

 end. Soon the notochord is found fused 

 to the hypoehordal rudiment; and as the whole mass sinks further 

 into the hypoblast, it becomes impossible to identify the hypoehordal 

 cells. Behind the section described the same changes supervene as 

 the posterior end is approached; and, of course, like those in front, 

 they are in inverse order to those that are taking place in actuality. 



In an embryo of 4 mm, through- 

 out almost the whole length of the 

 gut hypoehordal cells are apparent. 

 Except at the neuropore, the edges of 

 the neural plate have become ap- 

 posed; but the neurenteric canal is 

 not yet formed. 



Instead of beginning at the most 

 advanced part of the embryo, we will 

 begin near to the least advanced, 

 the "growing point" at the posterior 

 end, where the gut is, of course, 

 yet archenteric and with chorda- 

 hypoblast still in its roof. If a 

 transverse section a little in front 

 of the posterior end be examined, the following conditions can be 

 observed in the roof of this archenteric cavity (Fig. 2). A central 

 mass of cells with cytoplasm more deeply stained is flanked by cells 

 with cytoplasm more lightly stained. The nuclei of the latter are 



Fig. 2. 4 mm embryo. Through 

 posterior end. X 400. ch.hy chorda- 

 hypoblast. 



