82 RICHARD EVANS. 



{po. b.p.), Ilie foiutli somite {som.^) on one side and Uie fiftli on the 

 other {somJ>) appearing in it. The yolk protrudes out of tlie blasto- 

 pore {ex. y.). The central yolk \c, y.) is absolutely free of nuclei, while 

 the endoderm {eJi.) can be traced to the edge of the blastopore 

 {no. b.p.). The mesoblastic somites have no connection with the in- 

 vagination, which gives rise to the endoderm. 



Fio-. 10 g. — Tills figure represents the twentieth segment behind the one 

 shown in Fig. 10/. It passes through the anterior end of the primitive 

 groove. On the left side the rudiments of the seventh {som.') and 

 ei"-hth {som.^) somites are shown ; on the other side, owing to the 

 obliquity of the section, the mesoderm band {mesod.) is represented. 

 The external yolk spreads in a backward direction along the primitive 

 groove and ventral surface. 



Fi"-. 10 h. — This figure represents the fourth section behind the one 

 shown in Fig. 10_^. It passes through the primitive groove {pr. g) and 

 the individual mesodermal bands at their thickest part {mesod.). The 

 central yolk is still present and devoid of nuclei. 



Fig. 10 k. — This figure represents the eighth section behind the one 

 siiown in Fig. 10 h. It passes through the endoderm {en.) just at the 

 posterior limit of the central yolk. 



PLATE 7. 



Figs. 11 a — 11 d (x 120). — These figures represent four transverse 

 sections of the embryo shown in Fig. 3, in which rudiments of all the legs 

 have appeared. 



Fig. 11 a, — This figure represents the thirty-first section, from the 

 anterior end, of a transverse series. It passes through the first somite 

 (som.^), the inner wall of which has thickened considerably to form the 

 rudiments of the muscles of the stomodseum {stomod.), the anteriorly 

 directed loop of which appears in the section represented. The brain 

 {br.) has thickened considerably. 

 Eig. 11 b. — This figure represents the twenty-sixth section behind the 

 one shown in Fig. 11 «. It is slightly oblique, consequently it passes 

 through the second somite {som.^) on the right, and the third {som?) on 

 the left. The rudiment of the jaw is shown on the right side, while 

 the anterior edge of the rudiment of the oral papilla appears on the 

 left side. It shows the central yolk divided into masses, each of which 

 possesses a nucleus, a condition which should be compared with that 

 shown in Figs. 10 a — 10 k, in which the central yolk is devoid of nuclei. 

 The endodermal layer is distinct in both cases. 

 Fif. 11 c. — This figure represents the twelfth section behind the one 

 shown in Fig. 11 b. It passes through the third somite, that is the 



