928 



OF GENERATION EMBRYONIC DEVELOPMENT. 



continually narrowing hollow stalk (the vitelline duct) with the germinal sac 

 of the blastoderm which incloses the yolk. Soon after the appearance of the 

 primitive streak and groove the mesoblast undergoes a cleavage into two 

 larninse over its whole extent, with the exception of that part that lies just 

 beneath the axis of the embryo. The upper layer of the mesoblast unites 

 with the epiblast, the lower layer with the hypobltist, and the yolk thus 

 comes to have a double investment beneath the vitelline membrane. In the 

 first formation of the head, tail, and lateral folds, both layers are equally 

 involved, but, as shown in Fig. 329, the lower leaf is folded in more rapidly 

 than the'upper one, and a space the pleuroperitoneal cavity (pp, Fig. 329) 

 appears between them. The upper or outer layer forms the somatopleure 

 (F So), and subsequently gives rise to the body walls of the embryo. The 

 lower or inner layer forms the splanchnopleure (Sp), and its opposite folds 

 meeting sooner than those of the upper layer, constitute the intestinal tube. 

 At about the sixteenth hour a thickening of the mesoblast occurs in front 

 of the primitive groove, ending in front as the headfold (Fig. 328 A, Fig. 

 329). The surface of this is marked by a groove (m c, Fig. 328), termed 

 the medullary groove, the sides of which (A} constituting the medullary 

 folds or lamince dorsales, gradually arching over till they meet and fuse, in- 

 close the medullary canal. Immediately beneath the groove the cells form a 

 flattened cylindrical rod the notoclwrd, which is the precursor of the verte- 

 bral column. 



Formation of Amnion and Allantois. Coiucidently with the formation ot 

 the headfold, a second fold (Fig. 329, A m) appears a little in front of it, 



FIG. 329. 



NC. 



F So. 



Diagrammatic Longitudinal Section through the Axis of an Embryo. FSo, Fold of the somato- 

 pleure. FSp, Fold of the splanchnopleure. The line of reference, F S o, is placed in the lower buy, 

 outside the embryo. The line of D is placed in the upper bay, inside the embryo; this will remain as 

 the alimentary canal. Both folds (FSo, FSp) are parts of the headfold, and are to be thought of as 

 continually travelling onwards (to the left) as developuieat proceeds, pp, Space between somatopleure 

 and splanchnopleure; pleuroperitoneal cavity. A m, Commencing (head) fold of the amnion. The tail- 

 fold has not yet appeared. N C, Neural canal, closed in front, but as yet open behind. The section 

 being taken in the middle line, the protovertebra; are of course not shown. In front of the notochord 

 is seen a mass of uncleft mesoblast, which will eventually form part of the skull. D, Commencing fore- 

 gut, or front part of the alimentary canal. A, Epiblast. , Mesoblast. C, Hypoblast. 



and soon others arise at the tail end and sides of the embryo, which, unlike 

 the head-fold', are formed exclusively of the somatopleure i. e., of the epi- 

 blast and a thin layer of mesoblast, and gradually rising, bend over the 

 body of the embryo (Figs. 331, d, e; 334, a /), and coalesce (c, Fig. 

 332). These are the amniotic folds. Each fold necessarily consists of two 

 limbs. After their coalescence the septum disappears and two spaces exist: 

 one which is continuous with the pleuroperitoneal cavity between the outer 

 and inner limbs, and one between the inner limb and the body of the em- 

 bryo. The former is the space into which the attantoi* or respiratory organ of 



