INTRODUCTION TO THE HIGHER INVERTEBRATES 241 



oblique. As a result, the upper tier of four cells do not lie on top of 

 the lower tier, but are displaced circularly so that each upper cell 

 straddles two lower cells. The third cleavage is unequal and separates 

 four upper, small micromeres from four lower, large macromeres. The 

 four micromeres are called the first quartette. 



The fourth cleavage is also oblique, but always in the opposite 

 direction from the third (Fig. 13.2). The first quartette divides to form 

 eight cells. The macromeres divide unequally, producing an upper tier 

 of four micromeres, the second quartette, and a lower tier of four 

 macromeres. 



The fifth cleavage continues the pattern. It is oblique in the direc- 

 tion of the third cleavage (Fig. 13.2). The cells of the first quartette now 

 number sixteen. The second quartette divides to form eight cells. The 

 macromeres again divide unequally to produce an upper set of four 

 micromeres, the third quartette, and a lower tier of four macromeres. 

 As shown in the figure, the third quartette does not completely dis- 

 place the second quartette. Thus, the macromeres are ringed by eight 

 cells, the third quartette and the lower four cells of the second quartette. 



Divisions continue to be oblique, alternating in clockwise and 

 counterclockwise directions (often the first oblique division is counter- 

 clockwise, in which case the subsequent divisions are also reversed). 

 Ultimately, the cells formed from each cell of the four-cell stage lie ap- 

 proximately in the corresponding quadrant of the blastula. The animal 

 pole is occupied exclusively by the first quartette, the vegetal pole by 

 the macromeres, with the second and third quartettes somewhat inter- 

 digitated around the equator. 



Usually gastrulation begins after the sixth or seventh cleavage. 

 Without exception, the macromeres of the 32-cell stage or all their 

 progeny pass into the interior. Usually all of the mesoderm develops 

 from one of the macromeres, which is often slightly larger than the 

 others, while the other three macromeres become endoderm. The three 

 quartettes form all of the ectoderm. 



Variations from the pattern given occur in different species in all 

 of the phyla, but they are usually minor in degree, and the differences 

 between phyla are of no greater magnitude than those within phyla. 

 The presence of this pattern of cleavage in a series of phyla accounts for 

 the concept of a "main line" of evolution, proceeding from the flatworms 

 to the nemerteans, molluscs and annelids. In relation to this concept 

 some phyla, such as the Aschelminthes, show further specialization and 

 modification, while others, such as the chordates, show a loss or regression 

 to a simpler cleavage pattern. 



103. The Schizocoelomata and Enterocoelomata 



Looking forward to the remaining chapters, the student will find 

 that the eucoelomates are presented in two series, one including the 

 molluscs, annelids and arthropods, and the other including the echino- 

 derms, hemichordates and chordates. If the eucoelomates are divided 

 into taxonomic groups, these two series are the Schizocoelomata and 



