MUSEUM OF COMPARATIVE ZOOLOGY. 149 



PLATE in. 



Fig. 1. Abnormal? egg of fJ. /xir/na, in which the groove-like constriction which 

 generally encompasses the ovum and forms the first cleavage plane, 

 is limited to a furrow at one pole, p. 



" 2. Ovum in which this furrow, p, has deepened, forming a slit. 



" 3. Same egg in an older stage. 



" 4. Ovum in 2-cell stage, the original connection between the two cells in 

 the undivided part of the original ovum turned from the observer. 

 This egg is seen at right angles to the plane in which Figs. 1-3 are 

 drawn. 



" 5. An ovum in 4-cell stage, in which we have two large segment spheres 

 and two small. This condition is thought to be uncommon. 



" 6. Ovum in 4-cell stage, in which the beginning of the furrow destined to 

 divide each of the four cells is found on one side, inner side, of all 

 cells, and does not take the form of a groove-like constriction reaching 

 wholly about the blastomere. 

 7. Ovum in 4-cell stage in which each of the four blastomeres Is divided 

 later into two of unequal size. 

 8-12, Formation of the 3-cell stage. They first represent the formation of a 

 4-cell from a 2-cell stage, and then the subsequent breaking away of a 

 part of one segmentation or cleavage plane, so that two of the blasto- 

 meres are reduced to one. This is thought to be abnormal, patholo- 

 gical, or at all events unusual. 



PLATE IV. 



Fig. 1 Gastrula of Echinarachnius, in which the archenteron, ach, has made its 

 way to the ventral side of the body. Lateral view. 



2. The same, ventral view. 



3. An older stage in which the limestone rods, sp, have begun to form. 

 Lateral view. 



4. View of the last from anterior pole. 

 6. An older larva, showing the differentiation of the anterior lobe oud the 



posterior rods or arms. Ventral view. 



6. A slightly younger larva seen from one side. 



7. An older larva seen from one side. 



8. A larva older than the last, seen from the side. 



PLATE V. 



Fig. 1. Gastrula of E. parma. Lateral view. (The cilia on the body are too 

 faintly photographed.) 

 " 2. Older gastrula showing the " water-tube " at vp. Lateral view. As the 

 larva is under slight pressure, the spicule or calcareous rod, spi, is 

 slightly thrown out of position. 



