CHAPTER H 

 NEMIC EMBRYOLOGY 



B. G. CH1TWOOD 



Nemic embryology is a subject which has stimulated 

 much research, especially because of the fact that the 

 cells designed to form particular organs are laid down 

 in the very early cleavages. This type of development 

 is termed determinate cleavage and in substance means 

 that each blastomere may be identified in the egg as 

 the stem cell of a particular organ or part of an organ. 

 In other words, the fate of each cell is foreordained 

 from the first division. 



The regularity with which division takes place in 

 nematodes was observed by the earliest workers on the 

 subject. No attempt will be made to give an historical 

 account of the development of our knowledge other than 

 to point out a few of the steps. Biitschli (1875), Galeb 

 (1878), Goethe (1882), and Hallez (1885) were among 

 the pioneers in the field and to them the later workers 

 are indebted for breaking the ice, but in the light of 

 present day knowledge their observations appear rather 

 casual. The publication of Boveri in 1892 on the em- 

 bryology of Parascaris equoram was the foundation of 

 modern nemic embryology. His investigations were fol- 

 lowed by those of zur Strassen (1892, 1896), Spemann 

 (1895), Zoja (1896), Neuhaus (1903), Mueller (1903), 

 Martini (1903, 1909), and Pai (1927, 1928) as well as many 

 less comprehensive studies by other authors. It should be 

 stated that Boveri's work directly initiated the precise 

 study of the subject by later workers; all these investi- 

 gations have given us information equalled in few other 

 groups of animals. 



Nemic embryology consists of the study of individual 

 cells; in the early cleavages each cell is differentiated to 

 such an extent that it is capable of giving rise only to 

 certain parts of the organism; sister cells as a rule 

 differ to some degree in their potentialities. While there 

 is some difference in opinion as to what some particular 

 cells may give rise to in the mature organism, these 

 differences appear to be based more upon conceptions of 

 authors than actual conditions in given species. It is 

 not surprising that misinterpretations should arise in 

 the study of cell lineage where one must follow the 

 course of literally hundreds of cells. 



In nemas the development of germ layers as observed 

 in other animals is highly modified. In fact one can 

 hardly speak of germ layers in reference to nemas. In 

 the course of the first cleavages a number of so called 

 "primordial" or "stem" cells are formed (Fig. 151). 

 These are highly differentiated as to their potentialties. 

 ,Each of them will form a certain organ or organ system, 

 e. g., the anterior cell is destined to form the greater 

 part of the ectodermal epithelium and is designated SI, 

 which means first "somatic" stem cell; this cell there- 

 fore is the ancestor of the primary ectoderm. The 

 other cell, posterior in position, however, is a less dif- 

 ferentiated cell in its potentiality. It forms the remain- 

 der of the embryo; for this reason it is designated PI, or 

 first parental germinal cell, the fertilized ovum being 

 designated PO. This first cleavage is a transverse one. 

 These "primordial" or "stem" cells are therefore unequal 

 in their prospective potencies. 



Beginning with the second cleavage the cells of each 

 given family have their own cleavage "rhythm", that 

 is, cells deseendent from each primordial or stem cell 

 divide at the same rate but they often differ in the rate 

 from those descended from another primordial or stem 

 cell. Ordinarily one would expect this to be due to a 

 difference in the amount of yolk or the size of the cell." 

 but in nematodes this is not the case; instead the cell 

 has an inherent rate of cleavage without regard to sizp 

 or yolk and it cannot be explained as caused by mechan- 

 ical forces. 



The second cleavage is transverse in the SI cell forming 

 an anterior dorsal cell A and a posterior dorsal cell B. This 

 is followed by transverse division of the blastomere PI 

 forming an anterior ventral cell S2 and a posterior cell 

 P2. The second somatic stem (S2) cell is destined to 



form the somatic musculature, part of the esophagus, and 

 the entire intestine or mesenteron. 



At the third cleavage the SI cell group, A and B, 

 divides longitudinally forming four cells — two on the 

 right and two on the left side of the embryo; the cells 

 on the right are designated by Roman letters a and h, 

 while those on the left are designated by Greek letters 

 Alpha and Beta, for which the small capitals A and B 

 are substituted in the text. Following this the second 

 somatic stem cell, S2, divides transversely, the anterior 

 cell being destined to form the greater part of the meso- 

 derm of the body wall and esophagus, termed MST, and 

 the posterior cell the entire entoderm, designated E. The 

 undifferentiated stem cell P2 divides transversely, the 

 dorsal posterior daughter being termed S3 or the third 

 somatic stem cell, and the posterior ventral cell PS. The 

 third somatic stem cell is destined to form the ectodermal 

 epithelium of the posterior part of the body and is there- 

 fore .secondary ectoderm, it also forms a part of the 

 mesoderm in some nemas. 



The next or fourth cleavage is commonly said to com- 

 plete the formation of stem cells going into the forma- 

 tion of the soma or body. The cleavage of the SI group 

 is transverse forming a I, a II, a /, a //, b I, b II, B /, 

 and B //; of the S2 group M divides obliquely, transversely 

 or longitudinally forming either two cells one behind the 

 other St and M or two cells side by side mst and MST; 

 E divides transversely forming E I and E II; the S3 

 cell group divides longitudinally forming c and C; and 

 finally the P3 cell divides transversely forming PU, ventral, 

 and S' t , posterior. The descendants of Si are destined to 

 form the proctodeum or rectum and sometimes the meso- 

 derm or ectoderm of the posterior ventral part and 

 therefore either tertiary ectoderm, or possibly secondary 

 mesoderm (see description of embryology of Parascaris 

 equorum) . 



In further cleavages the descendants of given' cells 

 are designated by Roman and Greek letters (small 

 capitals in the text) if they go to opposite sides of the 

 embryo, i. e., a and A," by Roman numerals if serial, i. e., 

 / and //,- and by Arabic numerals if neither, i. e., 1 and 2. 

 Thus a longitudinal division of Si (D) forms cells d and D; 

 transverse division of them forms d I, d II, D I and D //. 

 Additional labels such as a', a" etc., are sometimes neces- 

 sary. PU is generally termed the primordial germ cell 

 but recent investigations indicate this may not be the 

 case. The epithelium of the reproductive system is not 

 germinal tissue but somatic tissue and is probably laid 

 down by a later cleavage. At the fifth cleavage two 

 cells are formed which are variously termed G I and G II 

 and S5 and P5; these cells appear alike. The earliest 

 differentiated genital primordium contains four cells, — 

 two epithelial and two germinal (G / and G //). The 

 former may be of mesenchymatous origin or they may 

 have been derived from the P line at either the fifth 

 or sixth cleavage. 



The various cleavages as outlined above do not take 

 place in all cells simultaneously so we do not have a 

 regular doubling of cells. Though the cleavage is total 

 or holoblastic, it is usually unequal; since it is neither 

 radial nor spiral there is no typical morula stage. There 

 may or may not be a segmentation cavity or blastocoele 

 and when such is present it is usually of small size. As 

 Martini (1908) showed, the absence of a segmentation 

 cavity is a negative point being in these forms entirely 

 dependent upon the depth of the cleavage furrows and 

 the depth of furrows does not appear to be correlated 

 with anything else in the cleavage of nemic ova and 

 embryos so that it must be regarded as without signifi- 

 cance. Generally speaking we may say that the blastular 

 stage begins in the 12 to 16 cell stages of the embryo 

 since it is at this time that a blastocoele appears in 

 certain species, Parascaris equorum, Rhabdias bufonis, 

 and Nematoxys ornatus, while embryos composed of 

 homologous cells in the species Camallanus lacustris and 



216 



