624 



VETERINARY PHYSIOLOGY 



B. Development. 



I. Early Stages. 



As the ovum passes down the Fallopian tube it is surrounded 

 by cells of the zona granulosa, and these probably serve as a 

 source of nourishment and may prevent the ovum from becom- 

 ing attached till it reaches the uterus and absorption of the 

 cells is completed. Sometimes implantation occurs in the tube 

 and a tubal pregnancy may ensue. 



It is unnecessary here to describe the changes in the ovum 

 before or immediately after its conjugation with the spermato- 



a,vn.- 



Fig. 239.— Transverse Section of 

 more advanced Blastoderm, to 

 show Epiblast, Mesoblast, and 

 Hypoblast, formation of Neural 

 Groove and splitting of the 

 Mesoblast. 



Fig. 240. — Longitudinal Section 

 through Embryo to show it Sinking 

 Down into Ovum and the Formation 

 of the Amnion, am. In the Meso- 

 blast round, all., the allantois, the 

 blood-vessels grow out to form the 

 placenta. 



zoon, since they are so fully dealt with in all works on Biology 

 (p. 29). 



The mammalian ovum is holoblastic, that is, undergoes com- 

 plete segmentation, and forms a mulberry-like mass of cells (fig. 

 237, A.). The cells then get disposed in two sets, a layer of 

 small surrounding cells and a set of large central cells (fig. 237, 

 jB.). The former constitute the Ectoderm and take part in 

 forming the processes or jprimitive villi by which the ovum 

 becomes attached to the maternal mucous membrane. The 

 latter spread out at one pole to form the hlastoderra (fig. 238, A.) 

 and dispose themselves in three layers — the epiblast, meso- 

 blast, and hypoblast (fig. 238, B. and C). From these layers the 

 various parts of the body are derived as follows : — 



I. Epiblast. — Nervous system ; epidermis and appendages; 

 epithelium of the mouth, nose, naso-pharynx, and all cavities 

 and glands opening into them, and the enamel of teeth. 



