xvii PKOTOCHOEDATA 591 



nuclei of the cells of the outer layer become more dense and stain 

 with avidity (Fig. 429, B). 



This phenomenon appears to be the outward sign of a physio- 

 logical differentiation between the ectoderm and endodermal cells ; 

 and all the invaginated cells, those which were columnar on the flat 

 surface of the hemispherical blastula, and also those which are sub- 

 sequently invaginated and form the roof of the archenteron, retain 

 clear and vesicular nuclei, and differ from the ectodermal cells in this 

 respect. Further, the rapidly dividing cells at the edge x, which, in 

 consequence of the contraction following on completed karyokinesis, 

 are small and rounded, are of two kinds ; those which are destined to 

 be added to the ectoderm have deeply staining nuclei, those which are 

 destined to be added to endoderm have vesicular nuclei. Here, then, 

 we have proof that at the edge x we have a meristem like that of 

 a plant, i.e. a growing zone, from the cells produced by which, different 

 layers are differentiated. 



The processes which we have been describing lead to the formation 

 of a cup-shaped gastrula with a huge blastopore. As Morgan and 

 Hagen (1900) have shown, there is a general growth in length of the 

 embryo, due to divisions of cells scattered throughout both ectoderm 

 and endoderm. In virtue of this growth the cup becomes more 

 comparable to a thimble. 



The rudiment of the first definite organ now appears. This is 

 the neural plate (n.p, Fig. 429), the so-called "medullary plate," 

 which here, as in all the higher Vertebrata, is the first organ to 

 appear. The ectoderm cells on one surface of the thimble become 

 more columnar than the rest, and this surface becomes slightly 

 flattened, and so the neural plate, is formed. The appearance of the 

 neural plate settles definitely the question as to whether edge x corre- 

 sponds to the dorsal edge of the blastopore or not. It also proves 

 that the diameter of the blastopore and the long axis of the nerve 

 plate are at right angles to one another. 



The wide blastopore becomes then reduced to a narrow pore by a 

 new process of growth, which starts at the lower lip of the blastopore, 

 in the region corresponding to the lower limit of the flat surface in 

 the hat-shaped blastula, a region which we have already denominated y. 

 Here a meristem of dividing cells is formed, exactly like that which, 

 in a previous stage, existed at the edge x. By the activity of this 

 second zone an up-growth of the lower ventral lip of the blastopore 

 takes place, and all that is left of the blastopore is a small rounded 

 aperture near the dorsal surface. Fig. 430 is intended to show how 

 much of the endoderm and ectoderm of the gastrula is developed 

 from the walls of the hat-shaped blastula, and how much owes its 

 origin to the two zones of growth, x and y. The blastopore is closed 

 by the meeting in the middle line of two lateral folds of ectoderm 

 cells termed the neural folds (n.f, Fig. 431). These flaps originate 

 at the sides of the blastopore and extend forwards along the sides of 

 the neural plate. They meet first over the blastopore and last over 



