3o8 



ENDOCRANIUM, BRANCHIAL AND NEURAL CARTILAGES. 



The branchial bars are hard and elastic, and have the general appearance 

 and consistency of hyaline vertebrate cartilage. Chemically and histologically, 

 they are quite different from the fibro-cartilage of the endocranium, or of the 

 neural plates. There is apparently no invertebrate, outside of the arachnids, 

 that has a tissue comparable with it. The nearest approach to it, chemically 

 and histologically, is the muco-cartilage in Petromyzon. The branchial bars of 

 Limulus correspond to the gill bars of the post auditory region of vertebrates, as 

 we first indicated in 1889. In 1893, ^^ called attention to the surprising histo- 

 logical resemblance between these cartilages and those of Petromyzon, and still 

 later, 1896, it was shown that in abnormal Limulus embryos, one or more pairs of 

 appendages were invaginated, forming transverse slits along the sides of the head, 

 that resemble vertebrate gill slits or the lung books of the arachnids. (Fig. 

 183,^.) 



Fig. 210.- 



-Sagittal sections of Limulus embryos, showing successive stages in the development of the branchial 

 cartilages. After Patten and Hazen. 



Development of the Branchial Cartilages in Limulus. — The following 

 description is based, in the main, on what takes place in the operculum. The 

 cartilages in the other abdominal appendages develop somewhat later, but in a 

 very similar manner. 



In an embryo of three abdominal segments, there is no trace of the opercular 

 cartilage. (Fig. 210, A.) By the time five abdominal segments are developed, 

 the outer wall of the somite forms a thick ring of mesoderm around the base of 

 the appendage. The opercular cartilage makes its appearance as a transverse 

 plate of cells subtending the ring, with its distal end projecting into the cavity of 

 the appendage. (Fig. 210, 5.) 



In the next stage, C, where one gill leaf is developed on the first branchial 

 appendage, the cartilages have increased in size and now show the features that 

 characterize them so clearly in the later stages; viz: i, the cartilage cells are larger 



