Reproduction 



277 



Mesoderm. The vertebrate mesoderm is at first devoid of segmen- 

 tation and ordinarily contains no definite cavity (Fig. 215). At an early 

 embryonic stage, the mesoderm upon either side splits into two layers: 

 an outer lying against the ectoderm, and an inner lying against the 

 endoderm. The two layers remain connected, however, at the upper 

 and thicker region of the original sheet (Figs. 228, 229A). At about the 

 same time, this dorsal and thicker zone of the mesoderm develops trans- 

 verse fissures which divide it into a series of paired blocks (somites) 

 lying symmetrically on either side of the neural tube (Figs. 228, 230). 

 This segmentation begins in the anterior part of the embryo and 

 progresses backward just as, in Amphioxus, the mesodermal pouches 

 are formed successively from anterior to posterior. 



The process of segmentation involves only the upper part of the 

 mesoderm. As segmentation goes on, the space between the lower thin 

 and unsegmented layers on either side becomes wider — a space already 

 recognizable as the coelom bounded externally by a somatopleure 

 consisting of ectoderm and the outer sheet of mesoderm, and internally 

 by a splanchnopleure consisting of endoderm and the adjacent layer 

 of mesoderm. The mesodermal layers upon either side grow down to 



>INAL CORD 



•EPIMERE 



PRIMITIVE DUCT 



NOTOCHORD 



NEPHROSTOME 



:^^<SPLANC 



,— ^ NOC — 



PRINCIPAL DIVISION 



PRIMITIVE DUCT 



SUPPLEMENTARY DIVISION 

 NEPHROSTOME 

 HY POME RE 



AORTA 



^" T OUTER GLOMERULUS 

 Fig. 229. Diagrams showing the primary differentiation of the embryonic 

 mesoderm, and three stages (A-C) in the development of the primitive renal 

 duct and a pronephric tubule. The duct and each tubule connected with it arise 

 from the mesomere. Two types of glomeruli — outer and inner — become associated 

 with the pronephric tubules. (After Felix. Courtesy, Neal and Rand: "Chordate 

 Anatomy," Philadelphia, The Blakiston Company.) 



