140 THE AMPHIOXUS. 



notocliord in this period of development, we shall 

 take especial note of two historical points : first, the 

 morphology; and, secondly, the histological differen- 

 tiation. 



Taking the first as our starting-point, we will 

 first of all observe the condition of the notochord 

 in a convenient myotome of the body, then the growth 

 of the notochord at the posterior end, and finally 

 the condition in the anterior end of the body. 



In the transverse sections of the last stages of 

 the former period of development in the region where 

 the myotomes are completed we saw the oval noto- 

 chord transverse section still wedged in between 

 the cells of the mesenteron. In the next stages the 

 notochord is being pressed out of the wall of the 

 latter, though it still to a considerable extent con- 

 tinues to lie inside it (Figs. 132-139). In the region 

 of the later segments we still find the former con- 

 dition of the wedging (Figs. 138, 139). Still further 

 back we find the notochord even yet not sharply 

 distinguished from the mesenteron (Fig. 140). In 

 the neighbourhood of the latest mesoblastic somite 

 it passes over into the dorsal fold of the mesenteron 

 (Fig. 141), and this, too, becomes at last flattened 



