Embryonic Development; Differentiation of Tissues - 281 



remnants of the gill slits persist in the adult 

 body (p. 283). 



Despite the large number of species, the 

 Vertebrata are classified as a subphylum in 

 the phylum Chordata (p. 664). In addition to 

 the vertebrates, the phylum Chordata in- 

 cludes a number of less familiar animals, 

 which possess gill clefts and a dorsal hollow 

 nerve cord, but no true vertebral column. In 

 place of a vertebral column, primitive chor- 

 dates possess an unsegmented flexible rod- 

 like supporting structure, the notochord, 

 which occupies an equivalent position in the 

 body (Fig. 15-10). Vertebrate animals develop 



neural groove (Fig. 15-11, A,B), which runs 

 lengthwise of the embryo and is broader at 

 the anterior end (Fig. 15-8D). The neural 

 groove is bounded laterally by the neural 

 folds, which rise slightly from the surface of 

 the embryo. Posteriorly the neural folds en- 

 circle the blastopore (Fig. 15-8D), which is 

 called the yolk plug because it is blocked by 

 yolk cells. 



As development proceeds, the neural 

 groove sinks below the surface of the embryo, 

 and the neural folds come together along 

 the mid-dorsal line (Fig. 15-11B,C). This in- 

 vagination forms the neural tube, which will 



spinal cord, 

 body wall 

 digestive tract. 



BRAIN 



GONADS 

 ANUS 



Fig. 15-10. Amphioxus, a primitive chordate, sectioned longitudinally. Note 

 especially: 1, the notochord; 2, the dorsal nervous system (brain and spinal 

 cord; and 3, the gill slits. This animal cannot be classed as a vertebrate since 

 it lacks a segmented vertebral column. 



a notochord, but only transiently, during the 

 embryonic period, prior to the appearance 

 of the segmented vertebral column. In fact, 

 it is the possession of a notochord, as well 

 as of gill clefts and a dorsal nervous system, 

 that confirms the relationship between the 

 vertebrates and more primitive chordate 

 forms, and justifies placing all these animals 

 in the same phylum. 



Origin of the Nerve Cord and Notochord. 

 The nerve cord and notochord arise almost 

 simultaneously in the late gastrula period 

 (Fig. 15-1 1A-D). The nerve cord comes from 

 ectoderm, as will be described presently, 

 whereas the notochord, at least in some 

 forms, arises from a strand of endoderm that 

 buds off from the archenteron (Fig. 15-11, 

 B-D). 



The nerve cord first appears on the dorsal 

 surface of the gastrula. In the beginning it 

 is merely a shallow depression, called the 



give rise to the brain and spinal cord (Fig. 

 15-1 IF). The neural tube continues to sink 

 below the surface of the embryo, and soon 

 it becomes roofed over by surface ectoderm 

 that encroaches from the sides. At an early 

 stage (Fig. 15-1 IF), the cavity within the 

 neural tube establishes continuity with the 

 enteron cavity, although later this neuren- 

 teric canal becomes obliterated and the en- 

 teron breaks through more ventrally to join 

 an invagination from the exterior, forming 

 a new opening, the anus (Fig. 15-81). 



The neural tube gives rise to: (1) the en- 

 tire nervous system, including the brain, 

 spinal cord, and nerves; and (2) the special 

 sensory epithelia, such as the retina of the 

 eye. The brain develops from the anterior 

 portion of the neural tube (Fig. 15-1 IF). 

 This anterior part is larger initially and 

 grows faster than the long narrow posterior 

 portion, which becomes the spinal cord. The 



