DEVELOPMENT OF ELASMOBRANCH FISHES. 147 



is to be regarded as the active layer of the epiblast, and that 

 after this has become differentiated, an organ formed from the 

 epiblast is always a product of it. 



Muscle- Plates. 



The muscle-plates at the close of stage K were flattened 

 angular- bodies with the apex directed forwards, their ventral 

 edge being opposite the segmental duct, and their dorsal edge 

 on a level with the middle of the spinal cord. They were com- 

 posed of two layers, formed for the most part of columnar cells, 

 but a small part of their splanchnic layer opposite the noto- 

 chord had already become differentiated into longitudinal mus- 

 cles. 



During stage L the growth of these plates is very rapid, and 

 their upper ends extend to the summit of the neural canal, and 

 their lower ones nearly meet in the median ventral line. The 

 original band of muscles (PI. X. fig. 8 7?i. ^V), whose growth was so 

 slow^ during stages I and K, now increases wdth great rapidity, 

 and forms the nucleus of the whole voluntary muscular system. 

 It extends upwards and downwards by the continuous conver- 

 sion of fresh cells of the splanchnic layer into muscle-cells. At 

 the same time it grows rapidly in thickness, but it requires some 

 little patience and care to unravel the details of this growth; 

 and it will be necessary to enter on a slight digression as to 

 the relations of the muscle-plates to the suiTounding connective 

 tissue. 



As the muscle-plates grow dorsalwards and ventralw^ards 

 their ends dive into the general connective tissue, whose origin 

 has already been described (PL xii. fig. 1). At the same time 

 the connective-tissue cells, which by this process become situ- 

 ated between the ends of the muscle-plates and the skin, grow 

 upwards and dowmwards, and gradually form a complete layer 

 separating the muscle-plates from the skin. The cells forming 

 the ends of the muscle-plates retain unaltered their prmiitive 

 undifferentiated character, and the separation between them 

 and the surrounding connective-tissue cells is very marked. 

 This however ceases to be the case in the r^rts of the muscle- 



