FORMATIONS FROM THE HYPOBLAST AND THE MESOBLAST. 969 



furnishes the epidermal layer of the body, and is known as the horny 

 layer. The stratum corneum can be differentiated early from the Mal- 

 pighian network: from the first arise hairs, nails, feathers, etc. 



FORMATIONS FROM THE HYPOBLAST AND THE MESOBLAST. 



From the hypoblast there forms from above a cordlike arrangement 

 of cells, which is placed lengthwise under the spinal furrow the chorda 

 dorsalis (Fig. 376, II, III, e). In man it is relatively thin. It forms the 

 foundation of the spinal column, around which the substance of the 

 vertebras subsequently becomes so arranged that it pierces them like 

 a thread through a string of pearls. After its formation, the chorda is 

 soon surrounded by a double sheathlike covering. Further formations 

 from the hypoblast do not occur at this time; it lies as a thin layer of 

 single cells directly on the splanchnopleure. 



While, formerly, the chorda dorsalis was in general believed to originate from 

 the mesoblast, most observers at present, incline to the view that its develop- 

 ment takes place from the hypoblast. The chorda begins to form at the anterior 

 nodular swelling of the primitive streak, and grows toward the head. At first 

 it represents a tube (Kupffer's canal, chordal canal) which opens posteriorly in 

 the primitive groove, later breaking into the yolk-cavity. The chorda occurs in 

 ascidia as well as in all vertebrates, although during their development it soon 

 undergoes retrogressive changes. 



On both sides of the chorda, the cells of the mesoblast group them- 

 selves into cubical structures, always arranged in pairs one after the 

 other: primitive vertebra (primitive segments or somites, Fig. 376, II, 

 u; III, u; and Fig. 374, F, MS). The first pair of these represent the 

 atlas. At a later period a cellular cortical and a nuclear region can be 

 distinguished in each primitive vertebra. The body of the primitive 

 vertebra is used only in part for the formation of the later vertebra. 



The portion of the mesoblast that lies peripherally from the primitive 

 vertebras, the lateral plates (Fig. 376, II, S), produces through the 

 dehiscence of its cell-layers two lame lias, which, however, remain united 

 opposite the primitive vertebras through the middle plates. The space 

 thus resulting within the lateral plates is designated the pleuroperitoneal 

 cavity or the coslom (III, K). The upper lamella of the divided lateral 

 plate is closely applied to the ectoblast and is known as the musculo- 

 cutaneous plate, somatopleure (Fig. 376, III, x) ; the inner layer unites 

 with the hypoblast, and is designated the gut-fiber plate or splanchno- 

 pleure (III, y). On the opposed surfaces of these two plates there de- 

 velops the flat epithelium of the large pleuroperitoneal cavity. On the 

 surface of the middle plate turned toward the ccelom there remain 

 cylindrical cells, the germinal epithelium of Waldeyer, from which the 

 oviducts and the ova are developed. 



From the somatopleure, according to Remak, originate the skin and the 

 musculature of the trunk, as well as the vessels; according to His, only the 

 musculature of the trunk. According to both observers the smooth muscle of 

 the digestive tract is derived from the splanchnopleure. 



Especial emphasis should be placed on the views of His, who believes that 

 the vessels, together with the blood and connective-tissue structures, do not 

 arise autochthonously from the mesoblast, but that certain cells wander from 

 the margins of the germinal layers, between the epiblast and the hypoblast, to 

 form the structures named. They are not formed through the process of cleavage, 

 but are derived from the elements of the white yolk lying external to the situ- 

 ation of the embryo, and they are thought originally to have wandered into the 



