THE FCETUS. 893 



the extrafcetal parts particularly in view, and reserving the others until we 

 come to examine the development of the foetus. 



EXTEENAL LAYER. 



a. Intrafcet al parts. The external or sensitive layer (serous stratum- 

 epiUast) forms several parts belonging to the foetus ; these are : 1, The 

 epidermis and its dependencies, such as the hair, hoofs, horns, glands of the 

 skin ; 2, The central nervous system : the organs of the senses, such as the 

 retina and the labyrinth. 



b. Extrafcetal parts. So long as the foetus is represented by a kind of 

 shield at a given point of the blastodermic .layers, the external layer is 

 everywhere continuous, and extends to the inner face of the chorion and the 

 surface of the embryo without any line of demarcation. But the embryo 

 soon becomes inflected at its two extremities; the external layer of the 

 blastoderm is depressed so as to give rise to two layers : the cephalic hood, 

 which covers the Cephalic extremity, and the caudal hood, which envelops 

 the other extremity of the embryo. 



The lateral parts of the foetus also become inflected towards one another, 

 as may be proved by a transverse section; thus closing-in, laterally, the 

 thoraco-abdominal cavity of the young creature which, during this move- 

 ment, is more deeply placed in the serous layer. The latter soon comes into 

 contact with itself, above the back of the embryo. 



A short pedicle, the superior umbilicus, unites the two portions of the 

 blastodermic layer. This pedicle is not long before it becomes obliterated, 

 then breaks, when the foetus is found to be inclosed in an independent sac, 

 which is nothing more than the amnion. 



Fig. 419. 



i/V 



TRANSVERSE SECTION THROUGH THE EMBRYO OP THE CHICK AT THE CLOSE OF THE 

 FIRST DAY OF INCUBATION; MAGNIFIED ABOUT 100 DIAMETERS. 



ch, Chorda dorsalis; h, External serous, or corneal layer; m, Medullary position of 

 serous layer ; PV, Primitive groove between the dorsal laminae, R/ and m ; dd, 

 Intestinal epithelial, or glandular layer (mucous layer) ; wwp, Prevertebral mass, 

 in which the primary, or protovertebrae, are formed, and which is continuous 

 with the middle lamina, sp: wick, Fissure in the middle lamina, presenting the 

 first indication of the pleuro-peritoneal (thoraco-abdominal) cavity, and of the 

 subsequent division of the middle lamina into two layers. 



The sensitive layer is therefore resolved into two sections : an internal, 

 the amnion ; and an external, the serous vesicle (false amnion of Pander, the 

 serous covering of Von Baer), which is applied to the inner face of the zona 

 pellucida, and concurs in forming the chorion. 



MIDDLE LAYER. 



All the parts to which the middle layer gives rise, belong to the organs 

 of the foetus; the development of this layer will be more appropriately 

 studied in the chapter devoted to the foetus. 



