THE EMBRYO. 1155 



The epiblast consists of a layer of columnar epithelial cells, which, however, 

 are somewhat flattened toward the circumference of the germinal disk. It 

 forms the whole of the nervous system (central and peripheral), the epidermis of 

 the skin, the hairs and nails, the lining cells of the sebaceous, sweat, and mammary 

 glands, the enamel of the teeth, and the epithelial lining of the nasal passage 

 and of portions of the mouth and pharynx. 



The hypoblast consists, at first, of flattened epithelial cells, which subsequently 

 become columnar and even larger than those of the epiblast. It forms the epithelial 

 lining of the whole of the alimentary canal except the anus and part of the mouth 

 (which are developed from involutions of the epiblast)*, the epithelial lining of all 

 the glands opening into the alimentary canal, the epithelium of the Eustachian 

 tube and tympanic cavity, and of the trachea, bronchial tubes, and air-sacs of the 

 lungs, the epithelium of the bladder and urethra, and also that which lines the 

 follicles of the thyroid and thymus glands. The endothelial lining of the heart, 



FIG. 698. Section across the anterior part of the medullary groove of an early embryo of the guinea-pig. 

 (By Schafer. From Quain's Anatomy, 1890.) Folds of epiblast rising up on either side of the middle line, and 

 thus bounding the medullary groove, mg. Middle of medullary groove, hy. Hypoblast, which is in contact 

 with the medullary epiblast at the middle of the groove, but is elsewhere separated from it by mesoblast, m, 

 which has burrowed forward between the two primary layers. A cleft is seen in the mesoblast on either side ; 

 this is the commencement of the anterior part of the body-cavity. 



blood-vessels, and serous cavities is also of hypoblastic origin, while recent obser- 

 vations tend to show that the primitive red blood-cells are derived from the same 

 source. 



The mesoblast consists of loosely arranged branched cells, Avhich are sur- 

 rounded by a considerable amount of intercellular fluid, and which therefore 

 may be considered as resembling embryonic connective tissue. All the other 

 tissues of the embryo are developed from it, including the extra-endothelial 

 portion of the walls of the blood-vessels, the skeleton and voluntary muscles, the 

 connective tissues, the spleen, the generative and urinary organs (except the epithe- 

 lium of the bladder and urethra), and the involuntary muscles. 



FIRST RUDIMENTS OF THE EMBRYO. 



The primitive streak alluded to above is a very transitory structure, which 

 merely marks the direction of the embryonic axis, the embryo proper being 

 developed immediately in front of it in the following manner (Figs. 697 and 

 701): 



First, two longitudinal ridges, caused by a looping or folding up of the epiblast, 

 appear, one on either side of the middle line. These commence in the anterior part 

 of the area germinativa, where they are united, and extend backward, one on either 

 side of the primitive streak, gradually enclosing it, and thus converting the 

 blastopore into the neurenteric canal This folding up of the epiblast gives rise 

 to a lono-itudinal groove, the medullary or neural groove (Figs. 697 and ( 

 in consequence of the manner in which the cells of the epiblast are heaped up into 

 two longitudinal ridges, with a furrow between them so that the sides and floor o 

 the groove are formed of epiblastic cells (Fig. 698). The mesoblast fills up the 

 space between the epiblast and hypoblast, so that the sides of the groove are < 

 pied by a longitudinal thickening of mesoblast; the two longitudinal thicken- 



