IV. i INITIAL STRUCTURE OF THE GERM 235 



meridional and sagittal, the second meridional and transverse, the 

 third equatorial and horizontal. Since the animal pole will be 

 ventral, the vegetative dorsal, the right and left sides of the 

 future embryo are simultaneously determined. 



The four ventral animal cells consist almost entirely of clear 

 protoplasm, the two anterior dorsal entirely of grey yolk, the 

 two posterior dorsal almost exclusively of the yellow substance. 



The next furrows are, roughly, meridional and at angles of 45 

 to the first two ; they emphasize the bilateral symmetry, for in 

 the anterior half they meet the first furrow above, the second 

 below, and conversely in the posterior half. 



The direction of the divisions in the next phase may be most 

 easily gathered from the figures. In the vegetative hemisphere 

 four anterior chorda-neural cells are separated off ; the six 

 posterior cells include all the yellow substance and will give 

 rise to the posterior mesenchyme and the muscles of the tail ; six 

 endoderm yolk-cells remain, four anterior, two posterior. The 

 next division sees the separation of the chorda from the neural 

 cells in front, of an outside ring of muscle-cells from an inside 

 ring of mesenchyme cells behind, and of an anterior mesenchyme 

 cell on each side from the anterior endoderm. 



A few anterior animal cells become associated with the neural 

 cells in the formation of the nervous system ; the remainder of 

 this hemisphere is ectodermal. . 



In gastrulation the endoderm cells first sink inside, followed 

 by the chorda in front and the anterior and posterior mesenchyme 

 cells and the muscle-cells at the sides and behind, Overgrowth 

 is greatest at the anterior dorsal lip, the blastopore thus becoming 

 posterior. 



In normal development, therefore, the axes of the embryo are 



the 2-cell stage. The yellow protoplasm is indicated by coarse 

 stippling. (After Conklin.) 



a, 8-cell stage, posterior view, v, ventral ; D, dorsal, b, 16-cell stage, 

 anterior view. Normally, A 5.1 and a 5.3 lie in front of A 5.2 and a 5.4. 

 c, 16-cell stage, posterior view. In normal eggs the cells B 5.2 and b 5.4 

 lie behind B 5.1 and b 5.3. d, 30-cell stage, dorsal view, e, f, 34-cell 

 stage ; e, posterior, /, dorsal view. In normal eggs the cell B 6.4 lies 

 laterally to B 6.3, and the cell B 6.1 lies between B 6.3 and A 6.1. g, 46 

 cells: posterior view, h, 48 cells: dorsal view- The cells B 7.1 and 

 B 7.2 should lie next the middle line. 



