160 



DEVELOPMENT OF THE MUSCLES. 



and may even leave a more or less distinct space unoccupied by cells (protovertebral 

 cavity). This cavity has occasionally been noticed (Lockwood, Bonnet) to be 

 continuous laterally with the mesoblastic (coelomic) cleavage (fig. 189, B ; see also 



A Fig. 189. Two SECTIONS OF A SHEEP 



EMBRYO. (Bonnet.) 



A, shows the cavity within the proto- 

 vertebne. In B, the protovertebra on 

 the left side of the section is united with 

 the lateral mesoblast ; on the right side its 

 cavity also is continuous with the ccelomic 

 cleft in that mesoblast. am, amnion ; 

 n. c, neural canal ; p.v, protovertebra ; 

 ao, aorta ; p.p, pleuro -peritoneal space 

 (ccelom). 



fig. 139, p. 117), and it is 

 probably the morphological equi- 

 valent of the ccelom in this part 

 of the mesoblast. Whether there 

 be originally a cavity or not in it, 

 the protovertebra presently be- 

 comes filled up with cells and 

 then forms a fairly compact mass 

 of cells which are mostly irregu- 

 larly arranged, but externally 

 (next to the cutaneous epiblast) 

 become regularly disposed into 

 an epithelium-like plate of co- 

 lumnar cells. This is known as 

 the muscle plate, and when the 



inner part of the protovertebra becomes broken up as a distinct mass and joins 

 with the neighbouring protovertebrae to form the membranous vertebral column 

 (see below), the muscle plates still remain distinct : in them therefore the original 



Fig. 190. TRANSVERSE SECTION 



OF THE TRUNK OF A CAT EMBRYO, 

 SHOWING MUSCLE PLATES. 



(E. A. S.) 



m.p., muscle plate ; ao, aorta ; 

 rti.g. , mid-gut ; am, anmion ; w, 

 vesicle of Wolffian body ; w.d., 

 Wolffian duct. 



mesoblastic segmentation 

 continues to be exhibited. 

 They do not long remain 

 as a single epithelium-like 

 layer, for the extremities 

 of this layer fold sharply 

 round and become continu- 

 ous with a cell-stratum, 



which immediately lines the interna. surface of the columnar layer and forms an 

 inner muscle-plate (fig. 190). It is uncertain whether the cells of this inner muscle- 

 plate are derived from part of the columnar layer which has folded over, or whether 

 they spring from other cells of the protovertebra. Soon after their appearance as a 

 distinct layer of the muscle-plate they begin to elongate in the sagittal (antero- 



