CLEAVAGE OF LATERAL PARTS OE THE MESOBLAST. 



G93 



vertebra3 are not developed or are imperfect, it attains much larger pro- 

 portions, and itself constitutes the principal vertebral axis. 



Protovertebrae. — On either side of this axial cord a thick mass or 

 plate of mesoblast is collected along its whole length, and very soon 

 there appear several transverse clefts in these plates forming the com- 

 mencement of protovertebral segmentation. The first formed of these 

 divisions is near the anterior or uppermost of the future cervical vertebra3, 

 and they rapidly extend backwards in the posterior or lower cervical 

 and dorsal region (fig. 501, jw, and fig. 503, uic). The divisions 

 becoming more distinct, separate small quadrilateral masses, which 

 have received the name ol protovertebra, by which it is meant to indicate 

 that they are not the same with the permanent vertebral pieces of the 

 skeleton, but rather correspond to embryonic somatomes, or mciameric 

 segments, corresponding closely in number with the permanent vertebral 

 divisions, but including the rudiments of other parts, such as those of 

 the spinal nerves, along with those of the vertebra3. 



The basis of the cranium, into which the notochord extends, does not 

 at first present any transverse division similar to that of the vertebral 

 portion of the axis, and the notochord itself is at first without segmen- 

 tation, and forms therefore a simple and entire cylinder. 



Pleural Cleavage of the Lateral Parts of the Mesoblast.* — 

 Together with the formation of the protovertebral plates and their trans- 

 verse segmentation, another important change begins in the lateral 

 part of the mesoblast external to these plates, which consists in its 

 cleavage into an upper or outer and a lower or inner lamina, and the 

 consequent formation between them of an interval or space (figs. 

 503, sji, and 50-1, i^p). The two laminae thus separated constitute 

 respectively the somatopleure and s})lanchnopleure portions of the meso- 

 blast, and the space between them is the commencement of the pleuro- 

 peritoneal cavity, which afterwards forms by its partition within tl>e 

 embryo the pleurte, pericardium, and peritoneum, and which beyond 



Fig. 504. 



FSo. 



Fig, 501. — Diagrammatic longitudinal section through the Axis of an Embryo 

 (from Foster and Balfour) 



The section is supposed to be made at a time when the head-fold has commenced, but 

 the tail-fold has not yet appeared. A , epiblast ; B, mesoblast ; C, hypoblast ; FSo, 

 folil of the somatopleure ; FSp, fold of the splauchuopleure ; Am, commencing (head) 

 fold of the amnion : NC, neural canal, closed in front, but still open behind ; Oh, noto- 

 chord, — in front of it, uncleft mesoblast in the base of the cranium ; D, the commencing 

 foregut, or alimentary canal ; Ht, heart ; pp, pleuro-peritoneal cavity. 



* "Pleural" is here used in the sense " paiietaL 



