468 MALL. [Vol. V. 



Skeleton. 



The bulk of the framework in this specimen is composed of 

 multipolar cells. Although these play a very minor part in 

 adult higher animals, they are no doubt in embryos almost the 

 sole element which holds the tissues together. Besides these 

 there is the chorda dorsalis, which begins in the entoderm at 

 the base of the third branchial arch, and extends to the tip of 

 the tail. As the chorda passes between the two vertebral 

 arteries, it is surrounded by various compact groups of cells 

 which mark the bodies of the future vertebrae. On the oral 

 side of the first cervical nerve the group marks the occipital 

 bone ; it is the most conspicuous mass, extends from one side of 

 the body to the other, and sends two processes oralwards and 

 lateral to the two vertebral arteries. Next in distinctness is 

 the first cervical vertebra, while the following cervicals gradually 

 diminish ; the first three dorsal are faintly outlined. 



The Muscle Plates. 



The occipital cartilage is bound to the first cervical by means 

 of a muscle plate on either side. On the oral side of the occip- 

 ital there are three muscle plates lying over the three bundles 

 of nerve roots of the hypoglossal nerve. The plate nearest the 

 occipital is bound to the cartilage on one end, while the remain- 

 ing two have no cartilage between them. The plates gradually 

 become larger and larger as the dorsal region is approached, and 

 beyond this become smaller and smaller, until the tip of the tail 

 is reached. In all, there are thirty-eight in number. 



Especially well marked in the dorsal region are hollow out- 

 growths from the plates into the body wall. The rest of the 

 plate is solid, but from the aboral and ventral corner these 

 epithelioid prolongations extend beyond the beginning of the 

 body cavity. On the median side of each plate there is a 

 hyaline border which marks the beginning of muscle fibres. 

 Between this border and the nerve stem lies a group of cells 

 quite marked at the point the nerve ends, and gradually dimin- 

 ishing in number as the dorsal ganglion is approached. Many of 

 these cells, as they enter the hyaline border of the myotome, 

 become unipolar with the pole pointing towards the nerve root. 



