no PRINCIPLES OF EMBRYOLOGY 



the material for the chorda, more or less corresponding to this light-grey 

 crescent, which first becomes visibly distinguishable; it is important to 

 remember tliis difference between the two forms, which we shall fmd are 

 otherwise very similar in their general pattern. Above the ring formed by 

 the mesodermal and chorda-neural crescents hes an area which will form 

 ectoderm, and which in Styela can be seen to originate in the clear cyto- 

 plasm which emerged from the ruptured germinal vesicle; below hes the 

 dark-grey yolky material which develops into endoderm. 



It is now necessary to trace the movements and changes by which the 

 prospective areas attain their fate. These are perhaps slightly clearer in 

 Amphioxus (Fig. 7.3), but essentially the same features can also be seen 

 in the ascidians. The first cleavage plane, as has been said, lies in the plane 

 of bilateral symmetry which bisects the mesodermal and chorda-neural 

 crescents, and runs through the animal and vegetative poles. The second 

 plane is also vertical and is at right angles to the first, cutting off two 

 slightly smaller cells at the posterior side, and two larger ones in front. 

 (It was in regard to the orientation of this and the later stages that the 

 earlier workers were mistaken.) The next cleavage is horizontal, giving 

 an 8-cell stage in which the lower group of cells are all slightly larger than 

 the corresponding upper ones. Further than this it is umiecessary to follow 

 the cleavages in detail. As they proceed, a jelly-like material accumulates 

 in the centre of the mass of cells, which are gradually pushed outwards 

 to form a hollow sphere, which is a blastula of the typical form we have 

 already seen in the echinoderms. 



The gastrulation process, by which this blastula becomes converted 

 into a three-layered embryo, begins by a slight flattening of the vegetative 

 end. The cells in this region are those derived from the dark-grey yolky 

 material and are somewhat larger than any others in the blastula, so that 

 they form a rather solid-looking coherent flat plate. This sinks into the 

 interior of the hollow blastula. The opening which leads in from the 

 exterior towards the sunken plate is the blastopore, its edges the blasto- 

 poral lips. The cavity into which it leads, which grows deeper as the plate 

 sinks further into the interior, is the primitive gut. 



The shape of the blastopore alters somewhat as the gastrulation pro- 

 ceeds, but there is no need to repeat here all the details given by Conklin 

 in his classical account oi^ Amphioxus. At first the endoderm plate is tri- 

 angular, with a wide straight dorsal lip and two lateral boundaries which 

 converge towards the ventral lip. Although the sinking in of the endoderm 

 begins ventrally, it proceeds fastest on the dorsal side, and it is the dorsal 

 hp which is the most sharply inflected. In Styela this lip is originally made 

 up of about six transverse rows of cells, each row containing rather more 



