ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 81 



of tbe central nervous system are recognizable, arising in the outermost 

 ectodermic layer. A brief account of the histology of the system is also 

 communicated. Herr Eeichenbach maintains very strongly the primitive 

 connection between the nervous system and the other organs. The separa- 

 tion is a subsequent differentiation. Green-gland and third ganglionic 

 mass, eye and brain, &c., are primitively united. (3) The eye and ear. 

 The eye results from three factors, (i.) epidermal, (ii.) an ectodermic 

 invagination or eye-fold, (iii.) an optic ganglion. The epidermal layer 

 appears very early, rising on the head-lobe like an appendage, at first vs^ith 

 a single layer, afterwards with four or five. Its elements gather in groups 

 of eight elongated cells. Four of these become closely united and form 

 the peripheral covering and cuticular corneal facets. They are called 

 Semper's cells. The other four are the mother-cells of the crystalline 

 cones, which they form peripherally, while their inner processes are pro- 

 longed inwards to unite finally with portions of the eye-fold. The other 

 cells of the epidermal layer become the pigment-sheaths of the individual 

 eyes. The eye-fold appears as a flat groove on the head-lobes, is deeply 

 invaginated in the nauplius stage, and forms a solid mass of cells. It soon 

 forms two compact and complicated balls of cells, with the eye-fold long 

 persisting between them. The peripheral ball becomes united to the 

 processes of the crystalline cones ; its elements arrange themselves radially, 

 become grouped in sixes or eights, and form the retinula-cells, which form 

 internally the layer of rods. The inner ball comes into intimate connec- 

 tion with the outer and with the optic ganglion. The third factor or optic 

 ganglion arises in the nauplius stage as an ectodermal thickening in the 

 optic segment, in direct contact with the brain and the eye-fold. Its 

 details are then described. Eeichenbach emphasizes the analogy between 

 the Arthropod and Vertebrate eye-development. The invagination which 

 forms the auditory sac appears in the embryo with incipient abdominal feet, 

 but is not, of course, differentiated till afterwards. (4) The gills. Eatke's 

 results are simply confirmed. (5) The green-gland appears in the stage 

 with incipient walking legs as an invaginated sac on the basal joint of the 

 second antennfe. Its history is briefly traced through a few phases. 

 (6) The hind-gut arises in front of the closure of the blastopore as an 

 ectodermic invagination. Herr Eeichenbach gives a brief account of its 

 slight changes, and of (7) the greater modifications of the fore-g^it. 



Derivatives of the endoderm. — In this chapter Herr Eeichenbach traces 

 the endoderm from the circular disc sunk into the yolk and forming a 

 closed sac, to its final differentiation into mid-gut and liver. He devotes 

 special attention to the relation of the endoderm to the yolk, e. g. in 

 relation to the appearances known as secondary yolk-pyramids. Tho 

 usual bibliographic review closes the chapter. 



Derivatives of the mesoderm. — The heart is seen pulsating in the stage 

 with incipient walking legs. It lies under an arched portion of the 

 ectoderm which previously exhibited an accumulation of large loose meso- 

 derm-cells. This mesodermic rudiment is symmetrical ; the loose elements 

 unite to form the ventral wall, whence the sides grow up and meet dorsally. 

 The pericardium and mooring strands soon appear, and at a very early 

 stage the wall of the heart exhibits two layers. The hlood-vessels also arise 

 from wandering mesoderm-cells, which form strands, by-and-bye exhibiting 

 a lumen. The component cells are very small and flat. The development 

 of the main vessels is briefly noted. The Hood-sinuses are morphologically 

 persistent portions of the primitive segmentation cavity, as indeed are 

 also the cavities of heart and vessels. The elements of plasma or blood- 

 serum are primarily mesodermic, probably plus additional migrations from 



1887. G 



