78 SUMMARY OF CURRENT RESEARCHES RELATING TO 



proposal to rank them along with Myzostomiclfe in the class Stcleehopoda 

 is as yet warranted. (4) After balancing the arguments in regard to the 

 wider homologues of the Arachnida, he comimres spiders with insects. 

 The insect antennfe have no homologues, but embryonic labra correspond 

 to rostrum, mandibles to chelicerje, first maxillfe to maxillae, second 

 maxillae to first pair of legs, and then three pairs of legs are left to each. 



(5) So he compares the larva of Chilognatha with that of Acarina ; the 

 antennae of the former are again without counterpart in the latter, but 

 mandibles correspond to chelicerae, maxillae to maxillaj, three pairs of legs 

 to the three pairs of thoracic ditto, and lastly, the myriopod segments with 

 two pairs of appendages in the adult, to the abdominal zonites of the latter. 



(6) Schimkiewitsch maintains the Crustacean character of Limulus, and 

 regards the first two appendages of the latter as equivalent to the Arachnid 

 chelicerae and maxillae. The abdominal appendages of the Limuli are 

 represented by the embryonic abdominal appendages of Arachnids. (7) After 

 noting the ancestral character of PaiirojMS, the author sums iip in tlie 

 tabular survey which appears on the preceding page. 



Embryology of the Scorpion.* — Prof. A. Kowalevsky and M. A. Schulgin 

 have investigated the embryology of the scorpion, Androdonus ornatus. 



1. TJie differentiation of the germinal layers. — Segmentation begins in the 

 uterus ; no cells or nuclei occur in the yolk ; a single-layered blastoderm 

 is formed at one pole. In the middle of the lower surface of the layer some 

 cells are pressed in to form the beginning of the endoderm. On the 

 surface of the round germinal disc a round groove is formed, uniting the 

 central mass. In this an albuminous fluid gradually accumulates. The 

 margins of the groove grow up, arch over, meet a,Dd inclose the germinal 

 disc. Into this duplicature of the upper layer cells from the endoderm 

 penetrate. The inner layer of the embryonic membrane thus formed is 

 continuous with the endoderm and is the amnion proper, the outer layer 

 passes into the ectoderm, and is the serous envelope. Between the two a 

 few mesoderm cells can be detected. The round germinal disc elongates, 

 and one pole (the future post-abdomen) becomes thicker and longer, the 

 other (the future head) remains thin and broad. Meanwhile yolk-cells 

 from the disc wander inwards, become amoeboid, and dissolve the yolk mass. 



2. The formation of the alimentary system. — Ectodermic invaginations 

 form the fore and hind gut. The muscular sheath is formed from the 

 mesoderm. From the lower, properly speaking endomesoderm layer, the 

 true endoderm is separated otf as a thick row of cells closely apposed to the 

 yolk, round which the various layers then begin to grow. The endoderm 

 cells become filled with the yolk material, and form a thick inclosing 

 layer. When the yolk is completely surrounded, the post-abdomen 

 developes as an elevation composed of all the three layers. The endoderm 

 tube becomes connected with the hind-gut invagination. The liver lobes 

 are developed, and the mid-gut acquires floor, sides, and roof. 



3. Mesoderm and vascular system. — After the differentiation of the endo- 

 derm, the mesoderm begins to be differentiated. It remains longer than the 

 other layers below the germinal disc, and is last in growing dorsally over 

 the embryo. The number of mesoderm segments represents that of the 

 body. There is also a preoral segment with a cavity like the rest. The 

 somatic layer is much thicker than the splanchnic. They pass into one 

 another peripherally, and form on the margins of the body a complete 

 unsplit layer. 



The lateral margins of the mesoderm extend dorsally between ectoderm 



* Biol. Ccntrnlbl., vi. (188G) pp. 525-32. 



