456 



Mr. A. G. Huntsman. 



[Mar. 3, 



position of the original lumen is always indicated by the way in which the 

 cells are arranged. The anterior end of the tube is bent upward to a slight 

 extent (when the neuropore is open the anterior end is straight). 



When the anterior end of the neural tube separates into a dilated sensory 



vesicle (on the right side) and a narrow hypo- 

 physial canal (on the left side), it is the latter 

 that retains a connection with the ectoderm. In 

 this stage the anterior part of the canal is bent 

 into the form of a right angle (fig. 1b) and the 

 ventral part of the angle has sunk into the 

 endoderm. The endodermal cells are large and 

 columnar. They separate to make room for the 

 canal. Usually a distinct lumen can be seen in 

 the horizontal part of the canal but not in the 

 vertical part. Grouped around the latter are 

 mesodermal cells forming a characteristic ring. 

 The ectoderm at the neuropore has commenced 

 to invaginate and as the test is appearing the 

 invagination is easily distinguished. 



In the next stage (fig. lc) the hypophysial 

 canal has opened into the pharynx at the angle. 

 The margins of the opening in the wall of the 

 canal have become connected with the margins of 

 the opening made in the endoderm by the sinking 

 of the canal. This causes a great increase in the 

 lumen of the canal at the angle, where this 

 connection occurs. The vertical limb (which we 

 may now call the oral siphon) is still without a 

 lumen and the horizontal limb has little or no 

 lumen. The vertical limb or oral siphon has 

 become longer, partly from a lengthening of the 

 neural part and partly from a further invagination 

 of ectoderm. The layer of test has increased in 

 thickness. 



The fully-developed larva shows only unim- 

 portant advances on this condition. The oro-pharyngeal opening becomes 

 larger, the oral siphon longer and the layer of test thicker. The oral siphon 

 remains closed till after the metamorphosis. 



The above description and the figures are based upon the condition in 

 Dendrocloa, but the process is essentially the same in Clavclina and Ccesira. 



Fig. 1. — Diagrammatic sagit- 

 tal sections through antero- 

 dorsal part of Ascidian 

 embryos of three different 

 ages, showing transforma- 

 tion of neural tube into 

 wall of oral siphon. 



