ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 79 



and endoderm, in a still imsplit layer. The marginal cells separate off 

 from the others ; those nearer the back become round, sappy, and trans- 

 parent, like young ova. They move dorsally, and are the primitive blood- 

 corpuscles. They occupy a long broad cleft, which becomes narrowed by 

 the extension of the lateral mesoderm layers, which finally meet and fuse, 

 first dorsally, and after a while also next the endoderm. A mesocardium is 

 thus formed. The heart exhibits an inner endothelium and an outer 

 muscular sheath. The valves also appear. The alary muscles are also 

 mesodermic. Eound the heart, especially dorsally, large cells accumulate 

 and appear to form the pericardial membrane. 



4. The nervous system. — The first traces appear, when the head ap- 

 pendages become demonstrable, as ectodermic thickenings in the median 

 ventral line. Each joint of the body exhibits two elevations, (a) peripheral, 

 forming the appendages, and (&) median, forming a segment of the nerve- 

 cord. The ectodermal cells forming the nerve-strand proliferate, forming 

 in so doing peculiar pits or hollow cavities, which gradually disappear. 

 After considerable development, the fibrous substance becomes differen- 

 tiated, and the ganglia are then separated from the ectoderm. 



The cephalic nervous plate exhibits a paired hemispherical depression. 

 This insinking is siu-rounded by a ridge. The sunk portions form the two 

 cerebral masses, which remain for a while connected with the exterior by a 

 cleft. Above the closure a new elevation is formed, and two pouches are 

 produced, the first hints of the median eyes. In the sunk portion of the 

 nerve-plate which forms the brain, the formation of pits, characteristic of 

 the rest of the nervous development, is observed. 



The median eyes arise from the same primitive nerve-plate as the 

 brain. The peripheral portions of the fold associated with the develop- 

 ment of the latter form two lateral pouches. These two folds approach and 

 meet medianly, and when they meet the eyes are developed. The lateral 

 eyes have an entirely independent origin. 



5. The coxal gland was seen, when the nerve-cord had been separated 

 from the ectoderm, as a paired tube, opening at the base of the second (?) 

 pair of feet, and reaching internally to the anterior lobes of the liver. 



6. The genital ducts were formed (a) from an internal fimnel-shaped 

 tube developed from the splanchnic layer, and opening into the body- 

 cavity, and (b) from an external invagination forming the outer portion 

 of the ducts. 



7. The pulmonary sacs were first seen as simple invaginations of a space 

 rich in blood-elements. 



Microtelyphonidse.* — In connection with his studies on primitive 

 insect and myriopod forms, Prof. B. Grassi has described the anatomy and 

 histology of Kcenenia mirdbilis, representative of a new order Microtely- 

 pTionidee, and forming the much desiderated intermediate form between 

 Gigantostraca and Arthrogastra. The Microtelyphonidaa have lost the 

 branchiae, but have not yet acquired organs of aerial respiration. Prof. 

 Grassi gives a comparative survey of primitive Arachnid orders. 



e. Crustacea. 



Development of the Crayfish.t — Dr. H. Eeichenbach has . completed 

 his memoir on the development of the crayfish, which he began almost ten 

 years ago. His results are contained in a handsome monograph of about 



* Bull. Soc. Entomol. Ital., xviii. (1886) pp. 153-72. 



t Abhandl. Senckenberg. Natuif, Ges., xiv. (1886) pp. 1-137 (14 pis.). 



