288 Veterinary Obstetrics 



At a very early period, this nutritive relationship between the 

 mother and the fertilized ovum is established by two outgrowths : 

 one of the extra-embryonal somatopleur, to constitute the am- 

 nion ; the other from the splanchnopleur, to constitute the allan- 

 tois, both of which we shall describe later. 



The Nervous System 



We have already intimated that the neural groove of the blas- 

 toderm marks the beginning, location and direction of the future 

 cerebro-spinal nervous system. Consisting of thickened epiblast, 

 the neural folds increase in prominence, while the groove between 

 them deepiens and the summits of the folds approach each other 

 as segments of an arch, to finally meet and fuse above the groove, 

 converting it into a closed tube which, lined with ciliated epi- 

 blastic cells and filled with fluid, is to persist throughout the life 

 of the animal as the central canal in the spinal cord and as the 

 ventricles of the brain, while, from the deeper epiblastic cells in 

 the walls of the tube, are to develop the nerve cells and fibers of 

 the cerebro-spinal axis. Under normal conditions, the neural 

 groove grows rapidly in length and depth and, in the rabbit, its 

 lips have met and fused to constitute a complete tube by the end 

 of the ninth day. At this time one can distinguish the spinal 

 cord, the fore-brain, the mid-brain and the hind-brain. 



The cerebro-spinal nervous system of mammalian animals cout 

 stitutes the central organ, about which the other portions of the 

 embryo develop in more or less complete harmony. Any inter- 

 ruption in the normal development of the cerebro-spinal axis in- 

 terrupts or vitiates the proper growth of other parts of the body. 

 Should the neural groove become branched and double at its 

 anterior end, there forms in the embryo two heads instead of one, 

 constituting a double-headed monster or bicephalus ; (See Fig. 35) 

 or the fission may extend more posteriorly to constitute double 

 neck or chest. Should the posterior end of the neural groove 

 undergo division into two parts, we have a monster in which 

 the posterior parts of the body are double, while the anterior 

 may remain single and normal. Should two neural grooves form 

 side by side and in intimate contact with each other, there 

 may result a double monster, of two more, or less separate 

 bodies as in Figs. 126 and 127. Finally, the division 

 between the two neural grooves may be complete and \Ma 



