MUSEUM OF COMPAKATIVE ZOOLOGY. 53 



lateral wall may not be formed between two or more rows, which will 

 then merge into one. 



2. Origin and Development op the Individual, 



My studies on this subject, which were undertaken for the purpose of 

 showing the unity of the type of budding throughout Ectoprocta, have 

 been very fragmentary. 



Figure 72 (Plate IX.) has been introduced for the sake of orientation. 

 It represents a longitudinal vertical section through the peripheral part 

 of a stock of Lepralia Pallasiana. The body wall is thicker at the mar- 

 gin (marc/.), and gradually becomes thinner as one passes backwai'd. A 

 septum (sejj.) has already arisen cutting off the youngest zooecium from 

 the more proximal one, which contains a young polypide ; proximal to 

 this is another septum, and the distal end of a third zooecium. 



Nitsche ('71, pp. 445—456) has already well described the process of 

 forming the zooecium in Flustra membranacea. In fact, he has studied 

 the organogeny more thoroughly in many respects than I have. IS^itsche 

 ('71, p. 452) showed that the wall of the advancing margin of the colony 

 was composed of two layers of cells, — an outer, " Cylinderepithelschicht," 

 which secretes a cuticula, and an inner, " Spindelzellschicht mit anliegen- 

 den Kornerhaufeu." As the body wall, formed directly from these cell 

 layers is left behind by the advance of the margin, it becomes continually 

 thinner. "Die Cylinderepithelzellen der Wandung platten sich weiter 

 nach dem proximalen Ende zu ein wenig ab, besonders die der Unterseite 

 verkiirzen sich, die einzelnen Zellen riicken anseinander, die Zellgren- 

 zen werden undeutlicher, die Kerne jedoch bleiben deutlich erkennbar." 

 Vigelius ('84, p. 76) could not find the inner cell layer in Flustra, even 

 at the youngest stages, and consequently he believed that only one ex- 

 isted at the margin, and that this went to form the " Parenchym- 

 geivebe " of the adult. Ostroumoff ('86*, p. 336) seems inclined to doubt 

 the existence of any mesodermal layer at the distal portion of the bud- 

 ding zooecium in Cheilostomes, and Seeliger ('90, p. 580) has failed to 

 find in Bugula " eine zusammenhdngeiule dem Ectoderm dicht anlie- 

 gende Schicht von mesodermalen Spindelzellen." Both Osti'oumoff and 

 Seeliger, however, believe in the existence of isolated mesodermal ele- 

 ments at the budding end. 



According to my own observations, there is usually only one continu- 

 ous layer at the budding margin of the stock. Thus, in Flustrella 

 (Plate IX. Fig. 79) one can usually distinguish a continuous ectoderm, 

 but the mesoderm (jns'Jrm.) is represented by scattered cells only. At 



