ii FORMATION OF THE EGG 65 



microscopists by the name of sections, and examine it by means 

 of the microscope, we shall see that segmentation has resulted in 

 this. Just beneath the egg-shell there is a thin layer of cells, 

 one cell thick, which completely surrounds the egg. Inside 

 this coat of cells is the food yolk, with a few cells scattered 

 about in it at rare intervals, something like the raisins in a 

 plum-pudding. 



With the next process the formation of the young Myriapod 

 may be said to begin. A strip along the length of the oval- 

 shaped egg is thickened, and this thick mass of cells represents 

 the future ventral surface of the animal. The rest of the thin 

 layer of cells already mentioned just below the shell will form 

 the shell or exoskeleton of the future animal. The thick strip 

 of cells at the ventral surface has by this time split into 

 layers, so that, resorting to our microscope again, a section through 

 the short axis of the oval-shaped egg a transverse section 

 will show us 



1. The egg-shell. 



2. A layer of cells completely surrounding the egg, thin 



everywhere but on the ventral surface. This layer is 

 known to embryologists as the epiblast. The thick 

 part of the epiblast on the ventral surface gives rise to 

 the nervous system. 



3 and 4. Two layers of cells connected in the middle, along 

 the line of the thick strip, but separate elsewhere, and 

 not extending round the whole of the inside. These 

 layers constitute what is known as the mesoblast, and 

 give rise to the muscles and most of the internal 

 organs. 

 5. The scattered cells in the yolk. They are known as the 



hypoblast and give rise to the digestive canal. 

 After this point is reached the formation of the organs 

 begins. The segments are formed in order from before back- 

 wards. First the head, then the next segment, and so on. 

 When the number of segments with which the animal will be 

 hatched are formed, another process begins, and the tail end of 

 the animal, which can already be distinguished, is bent towards 

 the head. This is a process that takes place in many animals 

 besides Myriapods, and is called the formation of the ventral 

 flexure. Shortly after this the animal bursts the shell and comes 

 VOL. V F 



