1156 



EMBRYOLOGY. 



ings of mesoblast being at first separated at the bottom of the groove by the junc- 

 tion of the epiblast and hypoblast (Fig. 698). The groove becomes deeper 

 in consequence of the further growing up of the cells to form the ridge on 



either side. In this way the ridges eventually be- 

 come two plates, the laminae donsales or medullary 

 plates, which finally coalesce and form a closed tube, 

 the neural canal, which is lined and covered by 

 epiblast (Figs. 699 and 700). These lining and 

 covering layers of epiblast are at first in contact 

 with one another, but eventually become separated 

 by mesoblast which grows up between them. The 

 coalescence of the medullary plates first takes place 

 in the region of the future hind-brain of the embryo, 

 and then extends toward the cephalic and caudal 

 ends. The posterior extremity presents a rhom- 

 boidal appearance before the laminae close ; this has 

 been termed the sinus rhomboidalis (Fig. 701). 

 The epiblast which lines the neural canal is devel- 

 oped into the nervous centres, that which covers the 

 canal 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 

 restrictions 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, in 

 front of the internal opening of the blastopore, a 

 longitudinal groove forms in the hypoblast; this 

 groove becomes closed off from the roof of the future 

 enteron and forms a rod of cells which lies between 

 the hypoblast and the neural canal. This rod of 

 cells is known as the notochord or chorda dorsalis, 

 and when fully developed is composed of clear epi- 

 thelium-like cells enclosed in a homogeneous sheath 

 (Fig. 700). It is essentially an embryonic structure, though traces of it remain in the 

 centre of the intervertebral disks throughout life. The collection of mesoblastic cells, 

 which forms a thick longitudinal column on either side of the neural canal, is termed 

 the paraxial mesoblast, as distinguished from the outer or lateral part" of the meso- 



FIG. 699. Transverse sections 

 through the embryo chick, before and 

 some time after the closure of the 

 medullary canal, to show the upward 

 and downward inflections of the 

 blastoderm. (After Remak.) A. At the 

 end of the first day. 1. Notochord. 2. 

 Primitive groove in the medullary 

 canal. 3. Edge of the dorsal lamina. 

 4. Corneous layer or epiblast. 5. Meso- 

 blast divided in its inner part. 0. 

 Hypoblast or epithelial layer. 7. Sec- 

 tion of protovertebral plate. B. On 

 the third day in the lumbar region. 

 1. Notochord in its sheath. 2. Medul- 

 lary canal now closed in. 3. Section 

 of the medullary substance of the 

 spinal cord. 4. Corneous layer. 5. 

 Somatopleure of the mesoblast. 5'. 

 Splanchnopleure(one figure is placed in 

 the pleuro-peritoneal cavity). 6. Layers 

 of hypoblast in the intestines spreading 

 also over the yolk. 4X5. Part of the 

 fold of the amnion formed by epiblast 

 and somatopleure. 



Neural canal. 



Epiblast. *< 



Pleuro- 



Proto- Wolffinn peritoneal 

 vertebra, duct. cavity. 



Hypoblast. 



"^ 



Notochord. Aorta. Splanchnopleure. 



FIG. 700. Section across the dorsal part of a chick embryo of forty-five hours' incubation. (Balfour.) 



blastic layer. The paraxial mesoblast undergoes a series of transverse segmenta- 

 tions and becomes converted into a row of well-defined, dark, square segments or 

 masses, the protovertebrce or mesoblastic somites, separated by clear, transverse 

 intervals (Figs. 700 and 701). They first make their appearance in the region 

 which afterward becomes the neck, and from there extend backward along the 



