184 



STUDY OF YOUNG CHICK EMBRYOS. 



area. Immediately below the head is the pro-amniotic area -(Pro.am) which con- 

 sists of only two very thin layers of cells, the upper ectoderm, the inner entoderm. 

 By following through the series of sections it is easy to satisfy oneself that the two 

 layers of the pro-amnion are directly continuous with the Irke-named layers of the 

 embryo proper. At a little distance from the head, the lateral limit of the pro- 

 amnion appears, being marked by the appearance of the mesoderm between the other 

 two germ-layers. The edge of the mesoderm is sharply defined. The ectoderm has 

 formed also the thick wall, Md, of the medullary tube, which at this point is 

 completely closed and has lost its connection with the epidermis. There are no 

 distinct cell boundaries anywhere in the walls of the medullary tube at this stage. 

 The nuclei are oval, each with its long axis more or less nearly vertical to the 

 surface of the tube. Mitotic figures are frequent and occur always near the inner 



mes.G. 



Md. 



Ao.d. Ph. a. Ao.v 



FIG. 134. SECTION OF CHICK EMBRYO WITH EIGHT SEGMENTS. TRANSVERSE SERIES 642, SECTION 86. 

 a, Ventral thickening of ectoderm (part of oral plate). Ao.d, Dorsal aorta. A o.v, Ventral aorta. ,Ec, Ectoderm. 

 G, Ganglionic crest. Md, Wall of medullary tube, mid-brain, mes, Mesenchyma. Ph, Fore-gut. Pro.am, 

 Pro-amniotic area. X 100 diams. 



or free surface of the medullary wall; in other words, next the cerebral cavity. 

 The microscopic structure of the tube is similar throughout its whole extent. Under- 

 neath the brain is the fore-gut, Ph, somewhat crescentic in cross-section, and formed 

 of a single layer of epithelium, the entoderm, which is thinner on the dorsal, thicker 

 on the ventral side of the fore-gut, a difference which becomes more marked in 

 later stages. In the median ventral area the entoderm is somewhat thickened and 

 adjoins a similar thickening , a, of the underlying ectoderm. The two thickenings 

 are beginning to unite at present, but are still distinct and easily break apart. 

 Very soon, however, they fuse into a single lamina, which is known as the oral 

 plate and in which all trace of the double origin is lost. The ectodermal thicken- 

 ing, a, is depressed below the level of the ventral surface of the head. By the up- 

 growth of the tissues around it, the depression is increased, until in later stages it 

 appears as a deep invagination, lined by ectoderm, and the floor of which is formed 

 by the oral plate. The invagination is termed the stomodcEum^. and is destined to 

 form a large part of the mouth. The oral plate soon undergoes autolysis, and by 



