1 6: 



EMBRYOLOGICAL TYPES 



become irregular. As a result of cleavage a ball of cells or 

 morula is formed, and as the number of cells increases the 

 ball becomes hollow. The central cavity is the blastocoel, 

 surrounded by a single layer of cells which are smaller in the 

 future anterior region of the embryo, and larger posteriorly. 

 The embryo at this stage is a blastula. 



Gastrulation. — The posterior side of the blastula, where 



Fig. 68. — Amphioxus : early stages of development. 



A, early blastula, showing the blastocoel (b) ; B, late blastula ; C, 

 beginning of gastrulation, the ectoderm (ec) can now be distinguished from 

 the endoderm {en) ; D, gastrula with primitive gut-cavity or enteron (e) ; 

 E, late gastrula, showing the blastopore (bl) or mouth of the enteron ; F, 

 stage in which growth in length has occurred as a result of the activity of 

 the cells round the rim of the blastopore. 



the cells are relatively larger, becomes flattened, and at one 

 point (on the future dorsal side) actually tucked in beneath 

 the more anterior smaller cells. In this way a lip is formed 

 which soon extends right round the flattened region, which 

 sinks in towards the centre of the blastula. This process of 

 tucking-in is known as invagination, and the lip beneath 

 which this takes place is the rim of the blastopore. At the 

 same time as the flattened region is becoming invaginated, the 



