EMBRYO WITH EIGHT SEGMENTS. 



183 



Section through the Optic Vesicles (Fig. 133). The section is oval, the head 

 being flattened dorso-ventrally. Its outer boundary is a layer of cells, EC, consti- 

 tuting the ectoderm. The inner and outer surfaces of the ectoderm are marked in 

 the section by distinct lines. With higher powers the ectodermal nuclei are readily 

 seen; there are no cell boundaries, although the protoplasm is gathered into columns 

 and strands with clear spaces between. We have in fact to deal rather with a 

 syncytium than with a layer of cells. On the dorsal side the ectoderm shows a 



pro.a 



FIG. 132. LONGITUDINAL SECTION or A YOUNG CHICK EMBRYO. 



H, Head. Vd, Anterior portion of digestive canal (Vorderdarm). mes, Mesoderm. fo, Fovea cardiaca. p, 

 Pericardial coelom. pro.a, Pro-amnion. Ach, Entodermal cavity, in life bounded below by the yolk. Pr.s^ 

 Primitive streak. 



thickening, G. If this be followed back in the series of sections it will be found to 

 be continuous 'both with the ectoderm and with an internal group of cells alongside 

 the mid-brain (Fig. 134, G). We shall return to the consideration of the group in 

 question in connection with the description of the next figure. In the mid-dorsal 

 line the ectoderm from each side reaches the anterior neuropore, Np, which is still 

 open, and is reflected inward to form the thicker wall, Md, of the medullary tube, 

 here widely expanded to form the optic vesicles, Op. The outer ectoderm, EC, and 



FIG. 133. SECTION OF CHICK EMBRYO WITH EIGHT SEGMENTS. TRANSVERSE SERIES 642, SECTION 21. 

 EC, Ectoderm. G, Ganglionic thickening. Md, Wall of medullary tube. Np, Neuropore. Op, Optic vesicle, 



X ioo diameters. 



inner ectoderm, Md, are everywhere in contact with one another, so that in the 

 whole section there is but a single germ-layer, the outer. Soon the middle germ- 

 layer will penetrate between the two sheets of ectoderm, and permanently obliterate 

 the primitive relations. 



Section through the Mid-brain (Fig. 134). The section of the head is oval, and 

 bounded everywhere by the ectoderm, EC, or as it may now be called, the epider- 

 mis. The head is completely free, but underneath lie the layers of the germinal 



