120 ANIMAL BIOLOGY. [Parti. 



nucleus subdivides, the new nucleus shifting along the length of 

 the fibre. Thus is formed an elongated fibre containing several 

 nuclei. The protoplasm of the fibre, either alongside or around 

 the nuclei, becomes differentiated into the characteristic muscular 

 fibrils, only a small remnant of indifferent protoplasm remaining 

 around the nucleus as the muscle corpuscle. 



We have seen that the involuntary (unstriped) muscle of the 

 alimentary system is derived from the splanchnic layer of the 

 lateral plate. The main muscles of the trunk are derived 

 from the mesoblastic somites of the vertebral plates. 



Each vertebral plate originally contains a cavity, part of the 

 primitive body-cavity, by which it is divided into an inner 

 and an outer portion. That half of the inner portion which 

 lies next to the notochord and neural canal is utilised in the 

 formation of the permanent vertebrae. The other half of this 

 portion early develops into longitudinal muscles running through- 

 out the length of each mesoblastic somite ; and this, together 

 with the whole of the outer portion (which is also converted 

 into muscles), is known as the muscle plate. 



Cartilage. This tissue is developed from mesoblastic cells by 

 the formation of a capsule round each. A second capsule is 

 formed within the first, a third within that, and so on in orderly 

 succession, until a concentric system of capsules surrounds each 

 cell, the capsules first formed expanding as fresh internal capsules 

 are formed. The capsules so developed fuse together and blend 

 into the hyaline matrix of cartilage. The capsules are either 

 (1) secreted by the cells, or (2) formed by the differentiation of 

 the superficial layer of the protoplasm of the cell, or (3) devel- 

 oped in the intercellular substance around the cells. In the case 

 of elastic cartilage, elastic fibres are subsequently developed. 



Bone. While many of the bones take their origin by ossifi- 

 cation in cartilage, others arise by independent ossification in 

 connective tissue. The former are termed cartilage bones, the 

 latter membrane bones. 



The parietal of the rabbit, one of the roofing bones of the skull, 



