1!)()7.] NATllRAI. SCIEXCES OF IMIILADELrHIA. 521 



\-(»lk si)lu'i-ul(<s can !)(• seen iiisi(l(> the ('iiil)ry(). IW the eml of tlie 

 fiftli (lay the ciliated processes characteristic of the Miiller's larva' begin 

 to appear and the embryo exhibits fre(|ueiit contractions of its body. 

 On the sixth day the larva> bejiin to ])urst through the egg membranes 

 and to swim ai)out as typical Miiller's larva-. 1 did not succeed in 

 keeping these larva> more than two or three days, during which time 

 they seemed to undergo but little change. 



General Accouxt of the Kmbryoi/xjv. 



Following the example of many writers on embryology, it seems that 

 the later detailed account can be made briefer and more readily 

 understood if it is prefaced by a brief general sketch of the develop- 

 ment. The segmentation of the egg is total and slightly unequal. 

 From the first two divisions four cells result, of which one, the posterior, 

 is slightly the largest. Three quartets of micromeres are then given 

 off in alternating dexiotropic and lajotropic directions. The large cells 

 of the basal quartet then bud ofT at their lower, vegatative pole four 

 very small cells, which are to be regarded as the macromeres. The 

 large upper cells of this division form the fourth quartet. The large 

 posterior cell of this quartet, 4d, behaves very difTerently in its future 

 divisions from the other three. We may designate this cell as the 

 "mesentoblast," following Conklin's nomenclature. At the stage 

 with forty-fom- cells this mesentoblast buds into the interior of the 

 egg a large cell. 4(1'. Both of the mesentoblast cells then divide. In 

 these divisions the nuclear spindles lie horizontally. From the lower 

 pair of cells the greater part of the alimentary canal is derived. The 

 upper pair probably contribute a small amount to the alimentary 

 canal, while the larger portion goes to form the mesoderm of the body. 



In the later development the chief axis of the egg, i.e., the axis from 

 the animal to the vegetative pole, becomes bent, so that the animal 

 pole comes to lie at the anterior end of the embryo. 



From the first (juartet arises the ectoderm, covering the anterior 

 and dorsal jiortions of the body. From cells of this quartet four 

 strings of cells bud into the interior of the embryo and form the gang- 

 lion. The eyes arise in ectodermal cells of this quartet. The second 

 <iuartet gives rise to the larger portion of the ectoderm on the ventral 

 and posterior regions of the body. From cells of this quartet is formed 

 most of the ectodermal pharynx. A portion of the second quartet is 

 l)udded into the embryo and forms mesoderm. From this source 

 iU'ises probabh' only that mesoderm found around the l)lastopore and 

 which is later concerned in the structures of the pharynx. 



