114 EARLY STAGES IN THE 



The first change which occurs is the pushing in of one half 

 of the wall of the vesicle towards the opposite half. At the 

 same time by the narrowing of its mouth the hollow hemisphere 

 so formed becomes again a vesicle 1 . 



Owing to its mode of formation the wall of this secondary 

 vesicle is composed of two layers which are only separated by a 

 narrow space, the remnant of the segmentation cavity. 



Two of the stages in the formation of the secondary vesicle 

 by this process of involution are shewn in Plate X, fig. A II, 

 and A in. In the second of these the general growth has been 

 very considerable, rendering the whole animal much larger than 

 before. The cavity of this vesicle, A in, is that of the com- 

 mencing alimentary canal whose final form is due to changes of 

 shape undergone by this primitive cavity. The inner wall of the 

 vesicle becomes converted into the wall of the alimentary canal 

 or hypoblast, and also into part or the whole of the mesoblast. 



During the involution the cells which are being involuted 

 undergo a change of form, and before the completion of the 

 process have acquired a completely different character to the 

 cells forming the external wall of the secondary vesicle or 

 epiblast. This change of character in the cells is already well 

 marked in fig. A II. It is of great importance, since we shall 

 find that some of the departures from this simple mode of de- 

 velopment, which characterise other vertebrates, are in part due 

 to the distinction between the hypoblast and epiblast cells 

 appearing during segmentation, and not subsequently as in 

 Amphioxus during the involution of the hypoblast. 



Kowalevsky (Entwickelungeschichte des A mphioxus) originally 

 believed that the narrow mouth of the vesicle (according to 

 Mr Lankester's terminology blastoporc) became the anus of the 

 adult. He has since, and certainly correctly, given up this 

 view. The opening of the involution becomes closed up and 

 the adult anus is no doubt formed as in all other vertebrates by 

 a pushing in from the exterior, though it probably corresponds 

 in position very closely with the point of closing up of the 

 original involution. 



1 I have been able to make at Naples observations which confirm the account of 

 the invagination of Amphioxus as given by Kowalevsky, though my observations are 

 not nearly so complete as those of the Russian naturalist. 



