2OO GEORGE W. TANXREUTHER. 



large cell D (A-C, 4) divides first. The ectomere d l is budded 

 off in an upper anterior direction on the median dorsal side and 

 covers the posterior ends of the macromeres C, B and A. The 

 cells C, B and A before division are drawn out in an anterio- 

 posterior direction and divide nearly equal. The ectomeres r 1 , 

 & 1 and a 1 are on the same level with their parent cells, as shown in 

 the small diagram. During the fourth cleavage in the forma- 

 tion of the sixteen cell stage (A-C, 5) d- is the first cell formed, 

 which is budded off from D in a dorso-anterior direction. The 

 division of d 1 follows immediately. Next the macromeres C, B 

 and A with their ectomeres divide, thus producing a twelve, 

 fourteen and sixteen cell stage respectively. The embryo is now 

 composed of four rows with four cells each (A-C, 5). Jennings 

 describes the sixteen-celled embryo as being composed of four 

 layers, each layer containing a single derivative of each quadrant. 

 The first layer at the macromere end is designated as ventral 

 and the cells are labelled accordingly. The number of layers 

 increase with later cleavage. 



The cleavage forms of the male and the female embryos at the 

 sixteen cell stage are indistinguishable. The shape of the em- 

 bryos, the size of the cells and the direction of the various 

 cleavages (A-C, 5) are about the same. The only evidence we 

 have to distinguish the male embryo at this stage is the presence 

 of the two polar bodies, or more mature embryos in the same 

 uterus. In the fifth cleavage all of the cells except the macro- 

 mere D divide in a more transverse direction: thus doubling the 

 number of rows on the surface. The inner ends of the cleavage 

 cells withdraw towards the exterior and produce a central cavity- 

 which is later occupied by the large cell D. Before the fifth 

 cleavage is completed the anterior end of the cell D is partially 

 covered by the cleavage cell immediately in front of it. The 

 sixth cleavage is more irregular and doubles the number of cells 

 in each of the eight rows. 



Gastrulation is a double process, while the cells are passing 

 posteriorly over the macromere D, the cell itself is moving into 

 the central cavity. The entire process requires about fifty 

 minutes and can be demonstrated in the living egg. The large 

 cell D, which is now designated by the letter E, is destined to 



