FIRST RUDIMEXTS OF THE EMBRYO. 107 



greater part of these bodies and the outer layer of the amnion are also formed out 

 of the mesoblast. The foetal portion of the placenta, being essentially a vascular 

 structure, is also developed from the mesoblast. Its cells are irregular and 

 branched and surrounded by a considerable amount of intercellular fluid. It 

 may therefore be regarded as resembling more closely embryonic connective 

 tissue. 



First Rudiments of the Embryo (Figs. 79 and 80). The primitive trace alluded 

 to above as appearing in the area germinativa is a very transitory structure, which 

 marks the direction of the embryonic axis, and is gradually lost sight of as 

 development proceeds. 



The first real approach toward a definite form in the embryo is made (1) by 

 the development of the central nervous system ; (2) by the cleavage of the mid- 

 dle layer of the blastodermic membrane into a series of segments ; and (3) by 

 the development of an axial embryonic skeletal structure, the notochord. 



First, a folding up of the cells of the epiblast or outer layer takes place. This 

 commences in the anterior part of the area germinativa, and extends in the same 

 direction as the primitive trace, gradually enclosing this latter until it is lost 

 at the caudal extremity of the embryo (Fig. 81). This folding up of the epi- 

 blast gives rise to a longitudinal groove down its centre, 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 base of the groove are 

 formed of epiblastic cells (Fig. 82, A). The mesoblast, lying between the epiblast 

 and hypoblast. fills up the space thus caused between these two layers, so that 

 the sides of the groove are occupied by a longitudinal thickening of mesoblast ; the 

 two masses being separated at the bottom of the groove by the junction of the 

 epiblast and hypoblast at the situation of the primitive trace. The groove becomes 

 deeper and deeper in consequence of the further growing up of the cells to form 

 the ridge on either side. In this way the ridges eventually become two plates, the 

 laminie dorsales or medullary plates, which finally coalesce and thus form a closed 

 tube, the neural canal, lined by epiblast and having a covering of the same mem- 

 brane (Fig. 82). These membranes are at first in contact with one another, but 

 eventually become separated, mesoblastic structures growing up between them, and 

 the line of coalescence becomes obliterated. The coalescence first takes place in 

 the middle of the embryo, then toward the cephalic end, and lastly at the caudal 

 extremity. The lining of this tube is developed into the nervous centres, the 

 covering into the epidermis of the back and head. The cephalic extremity of 

 the neural canal is soon seen to be more dilated than the rest, and to present 

 constrictions dividing it imperfectly into three chambers : the brain is developed 

 from this dilated portion ; the spinal cord takes its origin from the remainder of 

 the tube. Below the neural canal the hypoblast and epiblast are in contact, 

 separating the two longitudinal thickenings of mesoblast on either side of the 

 canal. Here a thickening of the hypoblast, commencing from the anterior end of 

 the primitive trace, takes place, and gradually separates itself off from the hypo- 

 blast, lying between this membrane and the epiblast below the bottom of the neural 

 canal. This is known as the notochord or chorda dorsalis. This when fully 

 developed, forms a continuous rod-shaped body lying below the primitive groove 

 and composed of clear epithelium-like cells. It is essentially an embryonic struc- 

 ture, though traces of it remain in the centre of the intervertebral disks through- 

 out life. The collection of mesoblastic cells, which forms a thick longitudinal 

 column on either side of the neural canal, becomes separated from the rest of the 

 mesoblastic layer. It undergoes a series of transverse segmentations and becomes 

 converted into a row of well-defined, dark, square segments or masses, separated 

 by clear, transverse intervals, called the protovertebrceor mesoblastic somites. They 

 first make their appearance in the region which afterward becomes the neck, then 

 further forward toward the head, and afterward extend along the body. These 

 bodies, as will be explained hereafter, are not the same as the permanent verte- 

 brae, but they are differentiated, partly into the vertebrae and partly into the 



