598 



a single cell deep. At the same time the cells of the ganglionic layer, 

 without assuming such an extreme elongation as is seen in those just 

 mentioned begin to divide 'transversely, each parent cell giving rise 

 to a row of five or six daughter cells, each row growing inwards and 

 being directed toward the mathematical centre of the rudimentary eye, 

 thus giving the whole a radiate [appearance. The future history of 

 these cells need not be traced here ; it is sufficient to say that they give 

 rise to the series of ganglia and fibres which lie in the stalk, of the 

 adult eye, a union with the supra-œsophageal ganglion being effected 

 at about the time of hatching. As will be seen from the foregoing my 

 ganglionic layer (Anlage) corresponds, to a certain extent, to the neural 

 layer described by Bobretzky (cit. infra) in the developing eye of 

 Astacus. Up to this time the epidermal layer retains more of its pri- 

 mitive character. Its cells are large and flat and far less numerous 

 than those of either of the deeper layers. 



A new element now enters the eye. At the infero-posterior angle 

 a thin band of undifferentiated mesoblast pushes its way between the 

 ganglionic layer and the rudimentary supra-œsophageal ganglion until 

 it reaches'the point where the deeper layers of the eye separated from 

 the parent epiblast. It now turns upwards and extends in between the 

 ganglionic and retinal layers, thus coming to occupy the cavity of the 

 optic invagination. Here its remains for a time unaltered, its cells being 

 easily recognised by their slender fusiform outline. 



The next important change is noticed at the time when the depo- 

 sition of pigment in the eye begins. The embryo has now the same 

 number of appendages as before, but the distinctions between maxillae 

 and maxillipeds is well marked ; the abdomen is entirely formed ; the 

 telson is bifurcate and armed with its seven pairs of setae and the heart 

 has begun to beat. The cells of the epiderm have undergone a rapid 

 subdivision until now an epidermal nucleus rests directly over each 

 one of the rows of retinal nuclei about to be described. The elongate 

 retinal nuclei of the last stage have each been divided transversely into 

 five nuclei and each of these sets of products of a single nucleus are 

 arranged in a nearly straight line pointing, as did those of the ganglio- 

 nic layer, to the centre of the eye. From their subsequent fate we are 

 able to apply names to each of these cells indicated by these nuclei. The 

 outermost is the retinophora ^ and there in regular order come the pig- 

 ment cells 1, 2, 3, 4. The rows of cells are apparently arranged in sets 

 or pairs, a considerable interval of structureless substance alternating 

 with two rows of cells. This substance in reality is the rudiment of 



vide Patten, Mittheil. Neapel. 6. Bd. 1886. 



