92 MCMURRICH, [Vol. XI. 



In front of these another row of cells (7") is noticeable, consist- 

 ing of exactly eleven cells. This is the ectodermal teloblast 

 row, and it is always symmetrical, a central cell (cT) being 

 readily distinguishable with five cells on either side of it. At 

 this stage no further differentiation is noticeable, but a little 

 later the arrangement represented in Fig. ^6 is found. Here 

 a central, darkly-staining mass of cells may be found, which 

 consists of both mes-endoderm and vitellophags. It is several 

 cells thick, and represents the mesendodermal plug plus the 

 vitellophags oijaera. About the center of the mass a distinct 

 depression was visible in the specimen figured, but it does not 

 correspond to any deep invagination, but seems to be formed 

 by the withdrawal of several cells from the surface at this 

 point. The mesoderm is no longer distinguishable from the 

 endoderm either by its staining properties or by its arrangement, 

 but it seems highly probable that the anterior part of the mass 

 is mesendodermal, while the posterior is composed of vitello- 

 phags. What has happened is that the mesodermal row of 

 Fig. 35 has undergone a concentration towards the middle line 

 similar to what occurred in Jaera, a multiplication of the cells 

 having taken place at the same time. A careful examination 

 of the anterior edge of the mesendodermal mass shows a some- 

 what irregular row of eigJit cells {meT), a number which suggests 

 the possibility of these being the eight mesodermal teloblasts 

 which, as will be seen later, give rise to the meta-naupliar 

 mesoderm. The ectodermal teloblasts (7^) are even more dis- 

 tinct than in the last stage, and have arranged themselves 

 somewhat differently. The five cells of each side have come 

 closer together, and have separated from the central teloblast 

 {cT), and, furthermore, each of the lateral teloblasts has budded 

 off a small cell (^r'), the first of a teloblastic row. The lateral 

 teloblasts and their progeny form two rows each consisting of 

 five cells lying in front, and somewhat laterally to the mesoderm 

 plug. Extending from these rows laterally can be noticed on 

 each side a band of ectoderm cells, showing a slight tendency 

 to be arranged in rows, but nevertheless with a good deal of 

 irregularity. These two bands are the lateral ventral bands of 

 the naupliar region of the embryo. The cells which are to give 



