MUSEUM OF COMPARATIVE ZOOLOGY. 9 



nuclei are already arranged in a deep saiicer-shaped layer. The transi- 

 tion to the U-shaped arrangement of Figure 37 (Plate IV,), in which 

 the invagination of the inner layer of the bud is completed, is not a diffi- 

 cult one to understand. It is to be observed, however, that the folding 

 is of such character that it can hardly be termed a typical invagination. 

 Comparing Figures 4, 5, and 37, it appears rather to be of a type some- 

 what intermediate between typical invagination and typical ingi'ession. 

 The cavity of the bud first arises through a rearrangement and reshap- 

 ing of the cells of the inner layer of the bud. At this stage the nuclei 

 of the invaginated region stain very deeply, and have large nucleoli. 



Figure 21 (Plate III.) shows the condition of the bud at this stage as 

 seen in longitudinal section. The proliferation which gave rise to the 

 rudiment of the bud is shown, by a comparison of Figures 37 and 21, 

 not to have been confined to one point, but to have occurred along a line, 

 so that the resulting bud is boat-shaped, and not cup-shaped. The whole 

 mass is therefore bilaterally s^^mmetrical. Even at this early stage one 

 can distinguish a difference in the form of the bud at the anal and oral 

 ends. At the oral end (Or.) the bud passes more abruptly into the body 

 wall than at the anal end. Later, this feature becomes more marked. 

 This is an indication of a fact for which later stages will bring better 

 evidence : that the formation of the bud proceeds from the oral towards 

 the anal end, and that the increased length of the bud that one finds in 

 the stage represented by Figure 22 is due to growth at the anal end. 



4. Origin and Development of the Lateral Branches. 



The first lateral branch appears as a prominent protrusion of the lat- 

 eral walls of an individual of the primary branch when the ganglion of 

 that individual has already nearly closed, and when the bud of the next 

 younger individual has attained a stage somewhat later than that shown 

 in Figure 37. The zone in which the lateral buds arise lies about mid- 

 way between the neck of the median polypide and the tips of its tenta- 

 cles at this stage. The place of appearance in this zone is approximately 

 90° to the right or left of the neck of the polypide of the median indi- 

 vidual. In one case measured, however, that shown in Figure 20 (Plate 

 XL), the centres of the two lateral buds seemed to be unequally distant 

 from the neck of the polypide, and each over 90° from it (approximately 

 100° and 110° respectively. (Compare page 3.) 



A cross section of the branch through the region in which the lateral 

 bud is arising shows that the condition of the body wall at the bud is 

 quite different from that of the rest of its extent. Figure 19 represents 



