ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 67 



from au aunelid ancestor. There is a full discussion of the arguments 

 pro and con. the arachnidan affinities of Limidus. 



In conclusion, the systematic position of the Arachnida is treated 

 of, and it is thought likely that the ancestral Hexapod left the main 

 Arthropod stem some time before the separation of the Crustacea and 

 " Acerata " ; the characters of the common ancestors of the three, and 

 of the two latter groups are enumerated, and some of the arguments 

 in their favour briefly stated. 



Embryology of Limulus polyphemus.* — Messrs. W. K. Brooks 

 and A. T. Bruce describe the unfertilized ovum as consisting of a 

 homogeneous mass of yolk-globules, covered at one pole by a proto- 

 plasmic cap without a nucleus. From this, after fertilization, proto- 

 plasmic processes grow downwards, so as to divide the yolk into a 

 number of " yolk-balls," in which at present no nuclei are visible. 

 As the cap gradually decreases nuclei appear in it, and the yolk-balls 

 increase in number, whilst a nucleus appears in each of them. The 

 cap ultimately disappears. The cells at the surface of the egg now 

 become smaller and lose their yolks, becoming at the same time 

 columnar, so as to form a complete epithelial layer round the large 

 central yolk-containing cells. The former becomes ectoderm and 

 mesoderm, whilst the latter becomes endoderm, and perhaps also 

 mesoderm. The blastodermic cells give rise to a " protoderm," or 

 embryonic chitinous cuticle. A " primitive annulus " is formed from 

 the blastoderm, very like that which Balfour figures for the spider's egg. 

 The mesoderm grows in from between two ventral bands (nerve-cord) 

 and spreads internally ; this soon splits to form the coelom. The 

 yolk becomes segmented by mesodermic partitions. The entosternite 

 is formed by a thickening of the splanchnic mesoderm. The storao- 

 doeum is placed in front of the first limb-buds, and for some time 

 ends blindly against the yolk. When the embryo hatches there are 

 no endodermal structures. This layer is formed by the peripheral 

 cells of the yolk-mass becoming columnar and transparent. The 

 liver is marked off from the axial intestine by mesodermic upgrowths 

 from the entosternite. The proctodoeum does not appear till after 

 the formation of intestine. The commissure between the ganglia of 

 the first pair of appendages is in front of the mouth. The lateral 

 eyes are formed by a specialization of the ectoderm cells, but the 

 retinal portion of the median eyes is formed by ectodermal ingrowths 

 from the ventral mid-line; this development corresponds to the 

 difference in structure as described by Lankester for the eyes. 



The authors conclude by remarking that " the embryonic history 

 of Limulus finds its closest parallel in the embryology of the 

 Arachnida." They refer to Balfour's summary of the differences 

 in the development of the mesoderm in Arachnida and in the 

 Crustacea, in the difference in the mode and time of formation of 

 the mid-gut, &c., as tending to unite Limulus with the Arachnida. 



Metamorphosis of Limulus polyphemus. f — Soon after fertiliza- 

 tion the yolk of the egg becomes irregularly divided up, but Prof. 



* Johns-Hopkins Univ. Circ, v. (1885) pp. 2-4 (1 fig.). f Ibid., p. 2. 



F 2 



