82 PRINCIPLES OF EMBRYOLOGY 



The most vegetative cells begin to break loose separately into the blasto- 

 coel even before there has been much infolding of endoderm. These are 

 the 'primary mesoderm' cells, and they soon start secreting the fnst cal- 

 careous spicules of the skeleton. A similar process of shedding cells into the 

 blastocoel goes on particularly from the tip of the primitive gut as it 

 folds in; this is the 'secondary mesoderm', which later forms the greater 

 part of the skeleton and muscles. 



The pocket of endoderm pushed in to the blastocoel continues to elon- 

 gate, producing a long fmger-shaped primitive gut. The process is know^n 

 as 'invagination', a term v^hich is applied to all types of movement by 

 which the endoderm and mesoderm are formed from the blastula; as we 

 shall see, it covers several different sorts of foldings and cell migrations. 

 In the echinoderms, it appears to be a simple in-pushing, like that pro- 

 duced when one presses a thumb into a soft hollow ball. While this is 

 going on, the first signs of bilateral symmetry appear. The embryo, now 

 entitled to the name of gastrula, has become somewhat conical. It begins 

 to flatten on one side, which is the future ventral side. The elongating 

 primitive gut turns towards the upper end of this side, fuses with it, and 

 eventually breaks through to form an opening. This is the rudiment of the 

 mouth; and there is now a complete tube leading from it to the blastopore, 

 which from now on functions as the anus. The gut soon begins to differ- 

 entiate into an oesophagus, a stomach and an intestine; and meanwhile 

 four long arms grow out from the comers of the flattened ventral side, 

 while the opposite comer of the gastrula also elongates into a single thick 

 spike. This completes the formation of the pluteus. 



2. The gradient system 



Echinoderm eggs were some of the first with which experimentahsts 

 tried to solve the fundamental physiological problems of development. 

 They are easily obtained in large numbers; and it is simple to free them 

 of their membranes (by squirting them through a narrow pipette, for 

 example). Only their small size is an impediment; but this, so long a 

 major limitation on the kinds of experiment possible, was largely mastered 

 by the Swedish investigators Runnstrom, Horstadius and Lindahl, who 

 combined subtle chemical methods of attack with the most dehcate 

 manipulative skill. 



In the earliest experiments, which were mentioned on p. 62, Driesch 

 showed that if the first two, or first four, blastomeres are separated by 

 cutting along the cleavage planes, each isolated cell can form a complete 

 well-proportioned pluteus. He concluded that these cleavages do not 

 separate parts whose developmental fates have been already determined; 



