82 COMPARATIVE ANATOMY 



backward and around the posterior margin of the blastopore. Thus neural 

 plate and blastopore come to lie under a common roof of ectoderm and the 

 blastopore, no longer opening directly to the exterior, opens into the small 

 space between the neural plate and its newly acquired ectodermal roof. 

 The resulting relations of layers and cavities are shown in Fig. 55, a 

 sagittal section of an embryo at this stage. Upon conversion of the plate 

 into a tube, the blastopore is left in communication with the lumen of the 

 tube. At its anterior end the closure of the neural tube is delayed so 

 that for a time its lumen is open to the exterior by a small aperture, the 

 neuropore. The extraordinary result of these changes is an embryo 

 whose prospective digestive cavity, still devoid of definitive mouth and 

 anus, communicates via the neurenteric canal (the former blastopore) 

 with the hind end of the cavity of the prospective spinal cord and brain 

 and thence to the outside by the anterior neuropore (Fig. 55, P). These 

 relations, however, are merely temporary. Eventually neuropore and 

 neurenteric canal close. The definitive enteric apertures, mouth, gill 

 clefts and anus arise by very similar processes. At the appropriate 

 locality enteric endoderm and superficial ectoderm approach one another 

 and coalesce. The resulting double layer then thins out until perforation 

 occurs. 



The notochord, whose development is initiated by an upward folding of 

 mid-dorsal endoderm (Fig. 46D-F), early becomes detached from the 

 enteric endoderm and acquires its characteristic cylindrical form. The 

 enteric endoderm meanwhile closes in beneath the notochord and restores 

 the integrity of the dorsal wall of the enteron (Fig. 46G). As the embryo 

 increases in length the notochord grows within itself and receives acces- 

 sions from the active blastoporal region with which its posterior end 

 remains for some time connected (see Fig. 47). 



The more anterior mesodermal pouches (or somites), soon after their 

 formation and long before the more posterior somites have been developed, 

 begin to acquire their characteristic differentiation. The pouch expands, 

 especially ventralwards, and its cavity is correspondingly enlarged. That 

 part of its wall lying against the notochord becomes much thickened (Fig. 

 56) while elsewhere the wall remains relatively thin. The expansion of 

 the pouches continues until the walls of right and left pouches meet in 

 the median plane beneath the enteric endoderm. At this stage three 

 regions of the mesoderm may be distinguished: the thickened part lying 

 alongside the notochord; an outer thin layer contiguous with the ecto- 

 derm; and an inner thin layer similarly contiguous with the endoderm. 

 The thick part is destined to form a segment of body muscle and is there- 

 fore called the myotome (Fig. 56, M). The outer layer — being, in con- 

 junction with the ectoderm, the body-wall of the embryo — is called the 

 somatic or parietal layer. The inner layer, associated with the wall of the 



