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BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



pockets, like the lens and otocysts, become transformed into closed sacs ; 

 and grooves, like those of the lateral line, become transformed into canals. 

 In Invertebrates also this process is a very common one, being exempli- 

 fied in the closure of the blastopore, in the closure of the amniotic cavity 

 in Insects (K. & H., Figs. 474, 475, 484), and in the formation of optic 

 and otic vesicles generally (K. & H., Figs. 377, 630, G81-683). 



The end of the process of concrescence proper is a, fusion of the two 

 concrescing layers, whether the concrescence is occurring along free 

 edges, flatwise, or along the edges of folds. (Fig. 25, C, Fig. 24, II.) 



9. Frequently this process is followed by another one ; viz. the per- 

 foration of the fused layers (Fig. 24, III), or the separation of the 

 upper and lower components of the folds when two folds have been 

 concrescing (Fig. 25, D). By means of perforation the two spaces 

 separated by the fused walls are put into communication with one 

 another. By means of separation a pocket becomes a closed sac, and 

 a groove becomes a tube. 



IV. Ontogenetic Processes occurring in Protoplasmic Masses. 



These may be classed into three categories according as the most 

 prominent change produced is (IV a ) in volume, (IV b ) in form, or 

 (IV C ) in number of masses. 



IV a . 1. Under the first group are included changes produced by 

 growth which is not uniform in all parts. Thus the growth may be 



prevailingly along one axis, by which 

 & r .mai e.,*. means a cylindrical mass is derived 



from a spherical one (embryos of Dy- 

 ciemida?, K. & H., Fig. 99), or it may 

 be excessive at one pole (gemmules of 

 sponges, H. V. Wilson, '94, Jour, of 

 Morphol., IX., PL XVI.), or along one 

 meridian. Again the growth may be 

 more localized, being confined to a 

 small area or to a line ; as, for in- 

 Yig. 26. stance, in the case of the mesodermal 



core of the appendages of Arthropods 

 (K. & H., Fig. 371), and of Vertebrates (Figure 26, "limb-bud"). By 



Fig. 26. Cross section through emhryo of a Teleost, Fundulus, showing origin 

 of the pectoral limb-bud as a solid outgrowth of the somatopleure. After E. R. 

 Boyer, '92, Bull. Mus. Comp. Zool., XXIII. No. 2, Fig. 58. 



