DEVELOPMENT OF AMPHIOXUS 163 



rim of the blastopore is growing over towards the future 

 posterior pole of the embryo, a process known as epiboly. 

 Between them, the processes of invagination and epibolv 

 result in the conversion of the hollow single-layered ball (the 

 blastulaj into a double-layered hemispherical bowl. The 

 original cavity of the ball (the blastoccel) has been obliterated, 

 and the cavity of the bowl is the archenteron or primitive gut, 

 communicating with the exterior through the blastopore. 

 The embryo at this stage is known as a gastrula ; its outer 

 layer is the ectoderm which will give rise to the epidermis, 

 sense-organs, and nervous system ; its inner layer is the 

 endoderm which is destined to give rise to the lining of the 

 alimentary canal and its derivatives. The process of g-astrula- 

 tion therefore entails the separation of these germ-layers. 



The overgrowth of the rim of the blastopore or epiboly 

 continues as a result of the activity and division of its cells, 

 and produces an elongation of the embryo along its antero- 

 posterior axis. New cells are contributed to the ectoderm 

 outside and to the endoderm inside, and the blastopore 

 diminishes in diameter. The cells of the ectoderm develop 

 cilia, but the embryo is still enclosed within the vitelline 

 membrane. 



Mesoderm, Nerve-tube, and Notochord. — The cells along 

 the middle line of the roof of the archenteron are destined to 

 form the notochord. On each side of them is a band of cells 

 which will give rise to the third germ-layer, or mesoderm. 

 The cells along the mid-dorsal line of the ectoderm form a 

 flat band which sinks in beneath the surface, and is grown over 

 by the ectoderm on each side, which rises up to form the 

 neural folds. This flat band is the neural plate ; it soon 

 becomes V-shaped in section, and the two arms of the V join 

 so as to give rise to a long tube running all the way along the 

 back just beneath the ectoderm : the nerve-tube. In front, 

 this tube is open at the neuropore, a place where the neural 

 folds have not met, and which is indicated by Kolliker's pit in 

 the adult. Behind, the neural folds rise up at the sides of 

 and behind the blastopore. When they meet, they roof over 

 the blastopore, which thus no longer communicates direct to 



