292 C. O. WHITMAN. 



originally found on the floor of each somatomere, to the right 

 and left of the nerve-chain. The direction of the growth is 

 transverse along the inner wall of the body, from the median 

 ventral to the median dorsal line. 



IS. The closing of the germ-bands is followed by a bilateral 

 groivth of the mesoderm in two directions: — (1) around the ali- 

 mentary tract, meeting in the median dorsal line ; and (£) 

 around the nerve-chain, meeting in the median ventral line 

 [evolutio higemina, Baer). 



19. The sej)ta are diaphragmatic growths of the mesoderm 

 between the somatomeres. 



20. The entoderm (epithelium), which has its origin in the 

 peripheral entoplasts, encloses the i/olk remains as food. 



21. The diverticulate form of the digestive tracts is produced 

 by the antecedent growth of the mesoderm (septa, muscles, &c.). 



22. The circulator?/ apparatus consists of two systems : — (1) 

 a closed vascular system, and (2) a lacunar sifsiem. The lirst 

 has a dorsal and a ventral trunk, communicating anteriorly by 

 four pairs of branches and an odd pharyngeal branch, and 

 posteriorly by seven branches. The second has a median and a 

 marginal sinus, the former communicating with the latter by a 

 pair of transverse channels in each body-segment. 



General Considerations. 

 a. Axial Differ entation. 



The early "appearance of structural axes in the developing 

 Q^s^ is a very significant fact, and deserves special mention. 

 Among those authors Avho have called attention to this point 

 may be mentioned Auerbach (-v-.i, Ascaris and Strongylus), 

 Hatschek {Yuir, Pedicellina), Selenka (\--\, Holothuria), A. 

 Agassiz ( j-^T5 Cteno])hora), His ( "~ , Salmo), and Rauber (-firllj 

 bilateral animals). The unripe ovum is characterised by a 

 spherical symmetnj. At maturity, or immediately after, a main 

 axis of sijmmetrjj appears {radial sijmmtry^, and with, or during, 

 the cleavage, the lateral axes {bilateral symmetery^. This is the 

 order of axial difl'erentiation in bilateral animals. The first 

 indications of a main structural axis appear with the passage of 

 the germinal vesicle to the periphery of the q^'S^. The pole of 

 the axis thus localised marks, in some cases, the oral end of the 

 future embryo. The lateral axes, which in most cases becomes 

 recognisable during the cleavage, can be located in Clepsine 

 before the cleavage begins by means of the crescent shape 

 assumed by the upper ring-disc. At the completion of the 

 second meridional division, the right and left sides are given in 

 the lateral blastomer's {a, U). A complete bilateral symmetry 

 is established with the appearance of the neuroblasts and 

 mesoblasts. 



