HAS/C liEPRODnriVK AM) IIEh-KhlTY I'll ESuM HS A 10 



mult ici'llular aiiiiiials which n-prixhu'c acct)i(hiin to the ^!;aiiiic 

 iiH't hod (sexually) begin their existence as a single cell, the zyKotc. 

 After fertilization (synganiy) or the union of the two gametes 

 the resulting zygote (^singh^ cell) undergoes mitotic division. In 

 the division of the zygote this is referretl to as the cleavage or 

 segmentation of the egg. The resulting cells remain associated 

 in a cluster, and thus the multicellular state of the new individual 

 is attained. 



These cleavage or segmentation stages, which are preliminary 

 to the building up of the tissues and organs of the new individu:il, 

 can better be studied in such an egg as that of the starfish, in 

 which segmentation is ecjual and complete, than in the egg of 

 the frog, in which segmentation is complete but unecjual. Thf 

 principle, however, is the same in both. 



Development of the Starfish (Asterias). — Slides should be 

 furnisiied which contain at least seven stages in the ontogeny 

 (development of the individual) of this animal. Use low -power 

 only and locate the unfertilized egg wiiich can be recognized by 

 its large clear nucleus, containing a black spot, the 7iucleolus. 

 As it is often difficult to find such small eggs on a slide, it is 

 suggested that one focus with the low power on the edge of the 

 cover .slip. Then when the slide is moved so as to bring an 

 egg under the objective it will be seen. Having found this 

 initial stage in the development move the shde and with the aid 

 of the following descriptions identify and study each of the 

 stages : 



1. The unfertilized egg represents a single cell. The cytoplasm 

 antl nucleus with its small nucleolus should show distinctly; 

 also the egg membrane (or vitelline membrane). The colors 

 are due to the dyes used in staining the material in preparation 

 for study. 



2. After fertihzation the egg divides by mitosis. The first 

 division, the plane of which is meridional in direction, gives rise 

 to the two-celled stage, the two daughter cells remaining in con- 

 tact with each other, and \hc fcrtilizahOii nicmhraiic may be .^een 

 enveloping both cells. 



3. The second division occurs at right angles to the first l)Ut 

 also meridional in direction, resulting in the four-celled stage. 



4. The third cleavage plane appears at right angles to the 

 first two and may be spoken of as e(iuatorial in position. l-]ight 

 equal cells result. 



