186 



halves of the second furrow unite with the angles which this bent 

 portion makes with the rest of the first furrow (Fig. 3). 



In this way the »Querfurche« of Rabl or the »Brechungslinie« of 

 Räuber is formed. It will be observed that this »cross furrow« always 

 bends to the right when the first furrow is in the line of vision and to 

 the left when the second furrow is in that position. Since these rela- 

 tion do not change in the course of development and since the first 

 two furrows Avith the »cross furrow« are visible until near the closure of 

 the blastopore it becomes easy to orient these furrows with reference 

 to the axes of the embryo. In this way it is found that the first furrow 

 is transverse to the long axis of the embryo and divides the egg into 

 an anterior and a posterior half, Avhile the second furrow lies in the 



Fig. 3. 



long axis and divides the egg into right and left moieties. (In a former 

 communication already referred to a view the reverse of this was ex- 

 pressed the error having arisen from mistaking the vegetative for the 

 animal pole.) 



Each of the four macromeres formed by the first two cleavages 

 contains the elements of both ectoderm and endoderm altough only the 

 left posterior macromere contains mesoderm. 



The whole of the ectoderm is separated from the macromeres in 

 three divisions of four cells each. Soon after the ectoderm has been 

 thus separated and at the stage when there are 20 ectoderm cells the 

 left posterior macromere divides forming a primary mesoblast which 

 comes to lie at the posterior end of the second furrow and from which 

 all the middle layer is derived. The residue of the four macromeres 



