REPRODUCTION 



91 



If "anus" is to be defined as the posterior aperture of the digestive 

 tube — digestive in strict sense — then the term should not be applied to 

 the external opening of the cloaca but rather to the internal aperture 

 between rectum and cloaca. By this definition the proctodeal aperture 

 would ordinarily become the external cloacal opening but in some cases 

 would become an external anus (Teleostei) or an internal anus (frog). 

 If the posterior extremity of the adult mammalian digestive tube includes 

 a remnant of the embryonic cloaca, then the mammalian "anus" is in 

 reality a remnant of an external cloacal aperture. There is much con- 

 fusion and doubt about homologies in this region of the vertebrate and, 

 among authors, there is corresponding confusion of terms. 



The Mesoderm. In contrast to the early condition of the mesoderm 

 in Amphioxus, the vertebrate mesoderm is at first devoid of segmentation 



Fig. 62. — Transverse sections of embryos of the amphibian Ambystoma; A, earlier 

 stage; B, later and more posterior than A. a, archenteron; e, otic pit; en, endoderm; 

 m, mesoderm; n, notochord; nc, neural crest; ng, neural groove; np, neural plate; p, 

 primitive groove. (From Kingsley, Comparative Anatomy of Vertebrates.) 



and ordinarily contains no definite cavity. At an early stage it appears 

 as a somewhat compactly arranged but indefinitely delimited sheet of 

 cells rapidly spreading from its region of origin into the space between 

 ectoderm and endoderm. The layer becomes thickest dorsally alongside 

 the neural tube and notochord (Fig. 62). At an early embryonic stage the 

 mesoderm upon either side spHts 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 edge of the original sheet (Figs. 

 63, 64A). At about the same time the dorsal and thicker part of the 

 mesoderm develops transverse fissures which divide it into a series of 

 paired blocks (somites) lying symmetrically either side of the neural tube 

 (Fig. 63). This segmentation begins in the anterior part of the embryo 

 and progresses backwards just as, in Amphioxus, the mesodermal pouches 

 are formed successively from anterior to posterior. Figure 64B shows a 

 diagrammatic section of an amphibian embryo at this stage. Figure 65 



