122 MCMURRICH. [Vol. X I . 



actly sixteen times mid the cctoblasts tJiirty-two or probably 

 iliirty-tJiree timeSy before tJiey relijiqiiisJi their teloblastic mode of 

 division. 



What determines the cessation of the teloblastic mode of 

 division is a puzzle, but it is accompanied by very decided 

 changes in the appearance of the ectodermal teloblasts. This 

 difference is shown in Figs. 63 and 64. In the latter the ccto- 

 blasts are very large and readily distinguishable, while in the 

 former this distinction is entirely lost and they have the same 

 size as the remaining cells of the ectodermal rows. Bergh 

 ('93) has shown that in My sis this is accompanied by a change 

 of position of the spindle in the cell ; so long as the mode of 

 division is teloblastic the equatorial plate lies in front of the 

 middle of the cell, but when it ceases the plate lies at the 

 middle and the cell divides into two equal parts. This stage I 

 have not succeeded in finding, and can only state that a reduc- 

 tion in size of the teloblasts takes place, and that they, to- 

 gether with the mesoblasts and the ectoderm cells lying behind 

 them (the anus making its appearance in the center of these 

 cells), form the telson. 



In a transverse section through one of the anterior thoracic 

 segments of a CymotJioa embryo (Fig. 65) in which the limbs 

 {th) are represented only by ectodermal thickenings, one finds 

 immediately below each of the nerve ganglia {N) a mass of 

 mesoderm cells {cM) which are evidently the result of the di- 

 vision of two mesoderm cells which were budded off from the 

 mesoblasts which lie on each side of the middle line. Periph- 

 erally to these are found some scattered mesoderm cells 

 {mM) which represent that portion of the mesoderm which 

 will give rise to the mesodermal tissues of the limb ; in the 

 next sections these are somewhat more abundant, the similarity 

 to Fig. 62 being thus more pronounced than in the section 

 figured. Still more peripherally is to be seen a compact mass 

 of mesoderm {IM) which corresponds to the lateral masses 

 seen in the more anterior segments of Fig. 62. In a later 

 stage the arrangement of the mesoderm is very similar. Fig. 

 66 shows a section through the first thoracic appendage of 

 such a stage, and from it it will be seen that the limbs have be- 



