96 PRINCIPLES OF EMBRYOLOGY 



These at first extend round the gut which is being pushed in fi:om the 

 blastopore, but as the whole embryo elongates, the mesoderm bands 

 come to lie more or less vertically. Finally a mouth is formed, either by 

 the tip end of the gut breaking through the ectoderm to the exterior, or 

 in some forms by a constriction which divides the blastopore into two. 

 A trochophore larva of this kind is obviously only a minor variation 

 from the general structure of a gastrula, and the whole egg takes part in 

 its formation. 



Figure 6.2 



Direct development (without larval stage) in a spirally cleaving egg (the 

 freshwater oligochaete Ttibifex). The macromeres have cleaved to give a 

 compact mass of largish endoderm cells {En), while most of the micromeres 

 have formed small ectoderm cells lying on the other side of the egg [Ek). 

 The cell 2d has, however, produced two groups of 'stem-cells', each com- 

 prising one neuroblast and three secondary myoblasts ; these bud off a row 

 of neural cells and three rows of ectodermal muscle cells. Below each of 

 these columns of four are mesoderm-stcm cells derived from 4</, which 

 produce the accompanying mesoderm. Each column is known as a half- 

 germ-band {rk and Ik) ; they unite on the other side of the embryo to form 

 the head, and, as they grow, to give rise to the rest of the body. (After 



Penners.) 



In these forms with larval stages in their development, the formation 

 of the adult does not take place till considerably later; we shall not discuss 

 the processes by which this eventually occurs except to point out that the 

 ectoderm and mesoderm of the adult ultimately trace back to 2d and 4^. 

 In some spirally cleaving types, such as oligochaetes, the development of 

 the egg is 'direct', in that the adult is formed without passing through 

 any special larval stage. In these animals it is difficult to find anything 

 which can be called a normal gastrula stage. The 'somatoblasts' 2d and 4^ 

 each divide into a right and a left half; and the cells so formed become the 

 mother-cells from which a long string of daughter-cells is budded off. 



