MUSEUM OF COMPARATIVE ZOOLOGY. 58 
lateral wall may not be formed between two or more rows, which will 
then merge into one. 
2, ORIGIN AND DEVELOPMENT OF 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 (marg.), and gradually becomes thinner as one passes backward, A 
septum (sep.) has already arisen cutting off the youngest zoocium 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 zowciwm in Flustra membranacea. In fact, he has studied 
the organogeny more thoroughly in many respects than I have. Nitsche 
CTL, 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 Körnerhaufen.” 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 
verkürzen sich, die einzelnen Zellen rücken auseinander, 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- 
gewebe” 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 zoœcium in Cheilostomes, and Seeliger (90, p. 580) has failed to 
find in Bugula “eine zusammenhängende dem Ectoderm dicht anlie- 
gende Schicht von mesodermalen Spindelzellen.” Both Ostroumoff and 
Seeliger, however, believe in the existence of isolated mesodermal ele- 
meats 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 (ms’drm.) is represented by scattered cells only. At 
