THE MESODERM. 



2 3 I 



3. The Lateral Plates. The lateral end of each primitive somite retains its 

 connection with the proliferating surface cells that constitute the germ wall. As 

 the germ wall spreads over the yolk, it gives rise to the lateral plates, or that part 

 of the mesodermic segments lying lateral to the zone of appendages. The lateral 

 plates are merely lateral extensions of the somites, and like them each one con- 

 sists of somatic and splanchnic layers, and may enclose a coelomic cavity. The 

 somite is the product of the apical growth of the anal plate; the lateral plates 



are produced by the lateral extension 

 of the germ wall. The peripheral 

 margins of the lateral plates finally 

 lose their connection with the germ 

 wall, and the cell layers separating 

 adjoining coelomic cavities break down. 

 From the lateral plates arise the 

 extra embryonic blood corpuscles, 

 the cardiomeres, pericardial chamber, 

 and the longitudinal, haemal muscles. 

 Lateral Plates in the Frog. The 

 mesoderm forming the segmented 

 lateral plates of the post-thoracic 





Fio. 131. FIG. 132. 



FIG. 131. Section through the fourth segmental sense organ, s.o. 1 , of the thorax of an embryo Limulus, about 

 stage I, together with the adjacent margin of the germ wall with its mass of fiber cells, /.c.; a. to i., fiber cells 

 of the same stage more highly magnified; d.e.f., similar cells from the adult, in a free anrcboid condition, and in 

 fission; g., same sense organ with its lens-like chitenous thickening during the trilobite stage. 



FIG. 132. Clusters of fiber cells from the posterior, haemal region of the thorax, about stage N, showing 

 mode of division and their transformation into muscles. Limulus 



metameres plays such an important part in the development of the arthropod 

 embryos, that one might reasonably expect to find some trace of them in verte- 

 brates. With this object in view, specially prepared frogs' eggs were studied in 

 a strong oblique light. In this work I was aided by Mr. A. O. Kelley, a grad- 

 uate student at Dartmouth, and the figures and descriptions on this point were 

 worked out by us together, in 1906-07. 



A considerable number show two or three pairs of lateral plates; a very 



