32 EMBEYOLOGY OF THE LOWEE VEETEBEATES ch, 



become roofed in by. a process of overgrowth on the part; of its lip. 

 The most conspicuous factor in this process consists of backgrowth of 

 the anterior portion of the lip while the growth of the lateral portions 

 inwards towards the mesial plane (so as to narrow the opening 

 from side to side) is less and less active the greater the distance 

 from the anterior end, until finally, in the extreme posterior portion 

 of the lip, such growth as takes place is relatively inconspicuous. 



In the process of gastrulation in Amphioxus there are then two 

 distinct processes at work (1) a process of invagination or involution 

 of the large-celled portion of the wall of the blastula and (2) a 

 process of overgrowth, most pronounced in the case of that portion of 

 the gastrular lip which is originally anterior. By the agency of (1) 

 there are established the two primary cell-layers — ectoderm and 

 endoderm while by the agency of (2) there is formed the dorsal wall 

 of the embryo with its potential later developments such as central 

 nervous system and notochord. 



' It will be noticed that the originally anterior portion of the 

 gastrular rim, when it has completed its backgrowth, lies above, 

 dorsal to, the now greatly diminished gastrular opening or blasto- 

 pore. Consequently it comes to be spoken of as the dorsal lip of 

 the blastopore. 



Although, strictly speaking, the terms endoderm and ectoderm 

 are expressive of topographical relation and their use is permissible 

 only after the one layer has become at least partially invaginated 

 within the other, yet it should be carefully borne in mind that these 

 two primary layers have already become distinctly differentiated 

 from one another during the blastula stage, long before the process 

 of invagination begins. One might indeed go farther and say that 

 endodermal characteristics, e.g. richness in yolk, have already made 

 themselves apparent in the abapical portion of the egg even before 

 segmentation begins. 



This fact is of far-reaching importance as we shall find in other 

 Vertebrates that the histological characteristics of ectoderm and 

 endoderm become apparent not merely before the actual process of 

 gastrulation takes place but, it may be, completely independently of 

 that process. 



(2) Polypterus. — In Polypterus an early stage in the process of 

 gastrulation (Fig. 19, B) shows a well-marked groove encircling the 

 egg a short distance on the abapical side of the equator. This groove 

 marks the line along which involution of the egg-surface is taking 

 place and its adapical lip represents the lip of the gastrula. It is 

 to be concluded from a still earlier stage observed and drawn by 

 Budgett (Fig. 19, A) that the involution groove appears first in the 

 region corresponding to the anterior lip of the gastrula of Amphioxus 

 and gradually becomes extended at its two ends until complete. 

 This is of importance as betraying a tendency for the invaginative 

 activity to be accentuated in this portion of the gastrular lip and 

 diminished elsewhere. 



