230 EARLY STAGES OF ARTHROPOD AND VERTEBRATE EMBRYOS. 



6. The Mesoderm. 



The Sources and Kinds of Mesoderm. 



We may distinguish four sources from which the mesoderm takes its origin, 

 viz: a. the procephalic mesoderm, arising by delamination from the anterior 

 portion of the primitive cumulus; b. the axial mesoderm (primitive streak) from 

 the axial teloblasts; c. the mesoblastic somites, from the sides of the anal plate; 

 and (I. the lateral plates, from the germ wall. 



a. Procephalic mesoderm. In the procephalon the mesoderm forms a 

 single unpaired, thin- walled chamber, approximately coextensive with the cephalic 

 lobes. It arises by delamination from the anterior half of the primitive cumulus; 

 it is not divided into an axial cord, somites, and lateral plates, and does not 

 appear to be comparable with that in the trunk. (Fig. 123, B.} It disappears 

 without giving rise to any permanent organ or tissue, except the investment of the 

 stomodaeum and forebrain. 



b. The postoral mesoderm consists of the axial cord, somites and lateral 

 plates. These three subdivisions are present in the whole postoral region or 

 trunk, but they may be very unequally developed. A striking feature in the 

 arachnids and vertebrates is the large size of the somites of the midbrain region, 

 and the atrophy of the corresponding lateral plates. 



1. The axial cord arises from successive proliferations, located in the median 

 line at the posterior end of the body. After the first one or two segments are 

 formed, an unsegmented, axial cord appears, extending forward from the center 

 of the anal plate to the stomodceum. Its presence is indicated, either by a faint, 

 median shadow (Fig. 125, C.p.c.), or by a groove, or "primitive streak" (Fig. 128, 

 D b . pr.s.), or by a sharp depression, or telopore. (Fig. 140, t.p.) 



The axial cord gradually breaks up, from before backward, into a. the 

 ectodermic middle cord, from which arises the median nerve and the epithelium 

 of the canalis centralis; b. into the mesodermic lemmatochord, or notochord; and 

 c. into the primary germ cells. (See notochord.) 



The axial mesoderm shows little or no trace of segmentation and never con- 

 tains a ccelomic cavity. 



2. The Somites. The thoracic and abdominal metameres are formed as 

 paired, wave-like ridges on the anterior lateral margin of the posterior cumulus, 

 or of the anal plate. At the same time, a band of mesoderm cells separates from 

 the inner surface of each ridge, giving rise to a pair of mesodermic segments, which, 

 as fast as they separate from the outer cell layer, form hollow, thick-walled somites. 

 In the later stages, the posterior abdominal somites arise as segments of the wing- 

 like expansions of mesoderm formed beneath the ectoderm, on either side of the 

 telopore. (Fig. 130.) 



From the somites arise the longitudinal muscles and cartilages of the append- 

 ages, the endocranium, the genital and nephric ducts, and the nephric tissue. 



