A progenitor blast cell in the posterior of the newly hatched larva of Nippostrongylus brasiliensis yielded a large population of coelomic cells among which were 2 previously undiscovered coelomocytes, C5 and C6. The coelomocytes lay subdorsal and posterior to the genital primordium, C5 on the right and C6 on the left. Also, in the first-stage larva, 7 single seam cells appeared in the hypodermis on both the right and left sides arrayed in tandem along its length. Each seam cell (1–5, 7) went through 2 divisions with spaces maintained between the formed quartets. However, seam cell 6 underwent an unusual series of divisions resulting in the formation of a huge amoeboid nurse cell that enclosed a quartet of small cells in a vacuole; the quartet also was derived from seam cell 6. Ultimately, all the seam cells, including the nurse cells on each side of the larva, regressed and disappeared, except for the quartet cells, now released from their vacuole. These latter cells then remained dormant during the life of the free-living stages. During the process of seam cell development, coelomocytes 5 and 6 aligned themselves closely to seam cells 6 and their progeny; some attached themselves to and even partially penetrated the nurse cells at the level of the vacuole. At the time of the second molt and the formation of the early–third stage infective larva, tiny vesicles began to appear in the cytoplasm of coelomocytes 5 and 6. As vesicles increased in number, they aggregated into a mass at either the anterior or posterior pole of the cells. Coelomocytes 1–4 situated anterior to the genital primordium differed from coelomocytes 5 and 6 in that they accumulated much larger numbers of vesicles that remained discrete in the cytoplasm and concentrated extraordinary amounts of vitamin B₁₂ that was recognized as a red pigment filling the vesicles. However, no red pigment ever was seen in the vesicles of coelomocytes 5 and 6. On the basis of the very early sexual differentiation of larvae in the rat lung, it was determined that the infective free-living larvae from which they were derived, and which contained coelomocytes 5 and 6, were female; those lacking coelomocytes 5 and 6 were presumed to be male.