270 ARTHROPODS. 



As there are numerous ovarian tubes in each ovary, 

 and as the same process of oogenesis is going on in each, a 

 number of eggs are ready for liberation at the same time, 

 and are simultaneously discharged into the oviduct of each 

 side. 



The ovum is not only furnished with yolk from the nutri- 

 tive cells, it is also surrounded by a shell, which is formed 

 in most cases by the follicle cells lining the ovarian tubes. The 

 shell is firm and chitinous. It is pierced by one minute 

 hole (micropyle), or by several. Through a micropyle a 

 single spermatozoon finds entrance, sometimes (as in the 

 cockroach) after moving round and round the surface of 

 the shell in varying orbits. 



Segmentation. — The segmentation which follows fertilisa- 

 tion is described as superficial or centrolecithal, the centre of 

 the ovum being occupied by a core of relatively passive yolk 

 with scattered nuclei. The result of segmentation is a sphere 

 or ellipsoid of cells, and on the ventral surface of this, on 

 what is specially designated the " ventral plate," the embryo 

 begins to be mapped out. A long shallow invagination 

 represents the gastrula, and establishes the (endodermic) 

 mid-gut, and at the same time the mesoderm. 



One of the most remarkable facts in the development is 

 the formation of a double fold of blastoderm, arching over 

 the ventral embryonic layer. The internal fold is called 

 " amniotic," the outer " serous," from their resemblance to 

 similar enswathing envelopes in the embryos of the higher 

 Vertebrates. These folds do not, however, take any direct 

 part in the development of the embryo. The same is pro- 

 bably true of the yolk, for though it segments around the 

 nuclei which it contains, it seems to be eventually absorbed 

 by the mid-gut. 



Developtnmt of the Organs, «/c.— The epidermis is formed by the 

 ectoderm. From the epidermis arises the external chitinous cuticle. The 

 muscles arise from mesoderm, which originates in connection with the 

 invagination of the blastoderm, and is in great part divided into a double 

 row of mesodermic segments. The appendages appear as paired bud-like 

 outgrowths of the ectoderm and subjacent mesoderm. The brain 

 arises as a paired ectodermic thickening on the head. It is connected 

 at a very early stage with the double ventral nerve-cord, which has a 

 gimilar origin along the ventral surface. In both cases there is necess- 



