XVII 



PEOTOCHOEDATA 



619 



of the embryo (Fig. 448) ; it lies on each side of the pharyngeal wall, 

 wedged in between this and the ectoderm (Fig. 449). The posterior 

 mesenchyme is situated at the hinder end of the tail (Fig. 450). 



When the blastopore has been reduced to a pore on the dorsal 

 surface, the neural folds (often termed the medullary folds) make 

 their appearance (n.f, Fig. 446). These are a pair of folds of 

 ectoderm arising from the sides of the neural plate, meeting each 

 other in the middle line behind the blastopore, but diverging from 

 one another widely in front. As development proceeds they meet 

 each other in the middle line farther and farther forward ; thus 

 the blastopore is covered in, and, as in Amphioxus, a neurenteric 

 canal is formed. Then the neural plate is covered in and converted 

 into a neural tube covered by 

 ectoderm. 



The neural plate is wide 

 in front and gives rise to a 

 correspondingly wide section 

 of the neural tube ; this is the 

 brain-vesicle ; the remainder 

 of the neural tube is narrow 

 and constitutes the spinal 

 cord. For a short time the 

 front part of the brain- vesicle 

 communicates with the exterior 

 by a pore termed the neuro- 

 pore, corresponding to the 

 neuropore of the larva of 

 Amphioxus, and, like it, due 

 to an imperfect union of the 

 neural folds in front. The 

 neuropore, however, soon closes 

 (Willey, 1893), but reopens 

 at the close of larval life when the larva undergoes metamorphosis. 



The lar.va soon afterwards bursts its chorion and enters on its 

 free-swimming career, propelling itself like a fish by lateral blows of 

 its tail. For a complete exposition of its organisation (which is 

 taken for granted by Conklin) we must turn to the older work of 

 Kowalevsky (1866, 1871), who worked chiefly with Ciona intestinalis 

 and with Phallusia mammillata. 



In the fully developed larva a mouth is formed by a small 

 iusinking of the ectoderm in front of the nerve tube. This insinking 

 involves the spot where the neuropore was situated, so that the 

 brain-vesicle would open into the pharynx if the neuropore were 

 still open. This is the case in Clavelina (Willey, 1893), but in 

 simple Ascidians, as we have seen, the neuropore becomes closed 

 during free-swimming life, but reopens during metamorphosis. The 

 hinder end of the endoderm forms a solid cord underlying the noto- 

 chord ; it soon breaks up into wandering cells. On the dorso-lateral 



mch 



FIG. 449. Transverse section through the middle 

 of a gastrula of Cynthia partita. (After 

 Couklin. ) 



wic/i, mesenchyme lying at the sides of the gut ; n.f, 

 neural fold ; n.p, neural plate ; ph, cavity of pharynx. 



