456 PRINCIPLES OF EMBRYOLOGY 



than the rather special cases we have chosen to exhibit the details of 

 each process separately. Moreover, in the individuation of the nervous 

 system a process of induction is very clearly involved and the pattern of 

 the nervous system itself is partly derived from that of the underlying 

 mesoderm. This is perhaps rather a special feature v^hich is not found, at 

 any rate with such clarity, in the development of many other organs, 

 but it has the advantage that it offers opportunities for experimental 

 analysis which would not otherwise be possible. 



By the time the blastopore has become reduced to a narrow slit, gastru- 

 lation may be said to be complete (although in the immediate neighbour- 

 hood of the blastopore invagination of mesoderm will continue for some 

 time longer). The structure of the sheet of mesoderm which acts as the 

 inducer of the neural plate is at this time as follows. At its most anterior 

 end it is thin and stretches widely from side to side; this part lies in front 

 of the future notochord and is known as the prechordal plate. Posterior 

 to it is the main mass of the chorda-mesoderm, the dorsal part of which will 

 become notochord, the lateral part somites, v^th the intermediate meso- 

 derm and the side-plate mesoderm still more laterally. Overlying the 

 whole mesoderm is the ectoderm on which a neural plate is, or shortly 

 will be, dehmited by the appearance of the neural ridges. From anterior 

 to posterior the neural plate can be divided into four main regions. The 

 most anterior of these, wliich overlies the prechordal plate, will become 

 the forebrain (which is also commonly known as prosencephalon, but by 

 some authors (e.g., Lehmann) as the archencephalon, and by others (e.g., 

 Dalcq) as the acrencephalon). This develops into the two vesicles of the 

 forebrain, the second of which bears the eyes, and it becomes associated 

 with the nasal placodes. Posterior to it is the region which will become the 

 midbrain and hindbrain which are collectively known as the deuter- 

 encephalon and are associated with the ears. The anterior tip of the chorda 

 lies somewhere within tliis region. Further posteriorly is the spinal or 

 trunk region ; and finally the most posterior end of the neural plate con- 

 sists of material which is not truly neural at all but will form part of the 

 mesoderm of the tail. The neural plate also has a certain structure in the 

 transverse plane, hi the dorsal midline the tissue is rather thin, forming a 

 shallow groove. On each side of this lies the main bulk of the neural 

 plate, wliich will form the walls of the neural tube. At the two edges 

 are the neural ridges which will develop into the neural crest and even- 

 tually form pigment cells, parts of the spinal nerves and certain ectomeso- 

 dermal derivatives, such as some of the cartilages of the head. 



The question immediately arises how the pattern of regions arising 

 within the neural plate is related to patterns which may be present in the 



